Understanding my genset alternator

The most resent loss to small boat refrigeration support is the announcement from Rparts.com that they will discontinue operations of Refrigeration Parts Solution Effective June 30, 2010.

Click to expand...

RParts got back in business last year and have expanded their services.**www.rparts.com

Another good resource is Kollman Marine Refrigeration.****www.kollmann-marine.com

We have a custom refrigerator/freezer*by Sea Freeze, Bellingham, WA that was installed in 2007.* http://www.seafreezeinc.com/* It was expensive but we would do it again.* The 6 cubic foot refer has 4" of insulation with 3.5" in the door, powered by a Danfoss BD35 with it's own compressor area exhaust fan.* The freezer is 2.5 cubic feet with the same insulation.* They share a common wall with 2.5" of insulation.* The freezer is powered by a Danfoss DB50, also with it's own compressor exhaust fan.* The*DB35 and DB50 compressors both use the same electronic module so spare parts are minimal.

With all this input I decided to do a little research. One, I need to probably redo my battery banks so that all 4 of the AGM batteries are on one bank and then install 2 engine start batteries in parallel on a separate bank. Maybe we can talk about the details on installing this later.

I've come up with 3 options to replace the power used. Cost is an important factor in deciding the best way to do this but not the only one. So without further ado here are the options I've come up with. Recall I use about 150 amps every 12 hours with my current refrig.

Note: Since I need to replace both alternators I am not including this cost, however upgraded alternators are included.

Option A: Replace both alternators, one with a 105 amp unit and the other engine with a 160 amp unit. The 160 amp unit will require some modification to the engine, but sbmar sells a kit to do this. The larger unit will go on the port engine because it is much easier to access. *I will keep the current Norcold refrig. The combined alternator ouput at 1000 rpm should be about 100 AH requiring a 1.5 engine run time every 12 hrs to get the 150 amps used. The cost of this option including 3 hours of mechanic help labor should be about $850. (Does not include the cost of the starboard alternator)

Option B: Replace both alternators with 105 amp units (cost not included). No engine mods necessary. Replace the Norcold refrig with a Isotherm CR219 or similar unit. Amp draw on this unit is about 65 amps per day. Ive upped the stated amp usage because Isotherm rates it at an ambient temp of 70 deg. I live in So FL. The alternators combine at 1000 rpm should produce about 70 amps. Using a refrig draw of 35 amp in 12 hours and other usage at 30 amps total out at 65 amps in 12 hours, requiring an engine run of nearly 1 hour twice a day or run the genny for a little more than 2 hrs. *Total cost is right at $2000, mostly for the refrig with a little for any extra wiring etc.

Option C: Replace both alternators with like units (no cost) and replace my current 30 amp 3 bank battery charger with a Magnum 2000 watt/ 100 amp inverter/charger. Keep the current Norcold. The generator would run about 1.5 hours twice a day to replace the 150 amps every 12 hours. I think the genset *would need to run closer to 2 hours with inefficiencies and such. *An added benefit is the inverter, but it is a modified sine wave. Cost of the charger/inverter is $1120, but estimating additional controllers and wiring could bring the cost closer to $2000. *Since the engines will only produce 70 amps it probably makes sense to just run the genny and not involve the engines. A Xantrex 2000 watt 100 amp pure sine wave charger/inverter is $1600.

Summary: Option A: Cost $850, Engine run 1.5 hrs every 12 hrs. Considerations; Old refrig may need to be replaced at any time. When refrig dies, engine run time reduced to :45 every 12 hrs or genny 2+ hrs.

Option B: Cost $2000, Engine run 1 hr every 12 hrs. Advantage; new refrig and should not need to be replaced ever. Adding one 160 amp alternator reduces engine run time to :45 every 12 hrs or run genny for a little more than 2 hrs per 12 hr charge cycle with the current 30 amp battery charger.

Option C: Cost $2000, Engines dont need to run, but genny runs 2 hrs per 12 hr charging cycling. Considerations; only option that adds an inverter. *In this option if the engines and genset/charger are running at the same time it would reduce the running time on each to less than one hour. Would it make sense to run both at the same time reducing the run on each?

An important consideration in all this is how we use the boat. I dont envision spending several days on the hook without moving the boat. I think perhaps 3 days at most and probably a day or 2 would be the norm. Running the engines with the current alternators would charge the batteries at about 100 to 150 amps per hour depending on how fast we run the boat. An hour or two of running like this will charge the batteries.

Option A of upgrading one alternator is by far the cheapest. When the refrig dies the cost goes to $2850 but that may or may not ever happen.

So what do ya think??

Tim

*


-- Edited by timjet on Thursday 1st of March 2012 12:47:47 PM

Run the old refrigeration until it dies.* Take the alternator off the gen and install it on one of the main engines.** Buy several cheap 100 amps battery chargers to multi gang charge the batteries and run the AC gen set for a couple of hour, estimate cost 200 to 500 bucks. *Buy inverter separate if you want/need one later,*estimate cost 700 bucks.

*

*

Tim:* Try plan C but instead of the Magnum 2000 watt inverter with 100 amp charger, go with the Magnum 2500 watt inverter with 125 amp charger.* The cost difference should be less than $250 and you get 20% more charging capability.* Keep*in mind when you run the generator you'll only get about 80-90% of the published charging amps from the charger.* I've never been a fan of using my main diesel to charge batteries at anchor.* Decisions, decisions, decisions...

Many small (under 10KW ) noisemakers will not power any large amperage battery charger to rated power,

Sure 100A or whatever was advertised might be possible on a 10.8V dead battset , but the power to charge 100A at 14.4 just isn't there.

A good H-D alternator is a different deal, IF it has a 3 stage smart charger.

Check on the existing noisemaker , you might get 2 belts to run a good alt and have minimum run time .

Remember you will be running between 85% SOC on the high side and 50% on the low with wet batts,.

Size your loads and batts to be happy in this 35% range.


-- Edited by FF on Thursday 1st of March 2012 03:28:12 PM

Larry M wrote:
Tim:* Try plan C but instead of the Magnum 2000 watt inverter with 100 amp charger, go with the Magnum 2500 watt inverter with 125 amp charger.* The cost difference should be less than $250 and you get 20% more charging capability.* Keep*in mind when you run the generator you'll only get about 80-90% of the published charging amps from the charger.* I've never been a fan of using my main diesel to charge batteries at anchor.* Decisions, decisions, decisions...

Click to expand...

*I never thought about running the engines to charge the battery, after all that's what the genset is for. However looking at the cost it can be cheaper to do just that. We did that all the time on the sailboat.

But as you say I'm not real wild about that either. We're going to check out the nose level and vibration with the engines running and sitting on the sun deck. To do that for 2 hours twice a day may just be too much. Good point.

Timjet,

I'm still having a little problem with a statement that you made on page two. "My twin '98 Cummins diesels are due for a front end alignment, according to the guru's on boat diesel. Meaning everything on the front of the engine that spins, idler pulley, alternator, and supporting stuff should be replaced. ".

I'm not a Cummins tech but statements like that*by mechanics sound like money makers and not*facts.* That being said.* If the guru was Tony Atherns then I might give it some credence.* How may hours are on your engines that they need to have everything replaced on the front end?* And why no matter how many hours?* Are there inherent failures? *My M/B has 150k on the odometer which is about 3,500 hours.*It has a 120 amp alternator on it and*I would not think of replacing everything on the front end of that engine.*There are trucks with Cummins*engines that have twice that many miles and I don't think that they have everything replaced just for drill.

Just been bothering me.

BTW I think you are on to what has to be done and there is no "one only way" to attack your problem.



--


-- Edited by JD on Friday 2nd of March 2012 08:03:15 AM

JD wrote:
Timjet,

I'm still having a little problem with a statement that you made on page two. "My twin '98 Cummins diesels are due for a front end alignment, according to the guru's on boat diesel. Meaning everything on the front of the engine that spins, idler pulley, alternator, and supporting stuff should be replaced. ".

I'm not a Cummins tech but statements like that*by mechanics sound like money makers and not*facts.* That being said.* If the guru was Tony Atherns then I might give it some credence.*

BTW I think you are on to what has to be done and there is no "one only way" to attack your problem.



-- Edited by JD on Friday 2nd of March 2012 08:03:15 AM

Click to expand...

*JD, that's very perceptive of you to pick up on that. The guru was indeed Tony and I just talked to him yesterday about what needs to be done.

I was incorrect in saying everything needs to be replace. The idler pulley has had a history of problems where the bolt that attaches the pulley to its mount has worn and in some cases has allowed the pulley to come loose. If this happens at high rpm, it can do big damage to the engine. Tony has talked to Cummins about this and they have made some minor changes, but not to Tony's satisfaction. He sells an upgrade kit for these pulleys that costs I believe less than $50. The other issue is the belt tensioner. It is pretty reliable from what I believe Tony said, but based on the age of my engines, '98, he suggests taking it apart, cleaning and re-lube them. The alternator is the other thing he thinks I should replace, primarily because of the bearings. If the alternator freezes up it could cause damage. Tony is a pro-active guy as you probably know and he knows what fails on these engines. I think he's trying to give me an heads up so I can prevent future problems. Besides he has no real skin in the game, but I think I will probably just send the idler pulley and belt tensioner to him and let him refurb them for me. Cost I believe was less than $100 per engine. Pretty cheap insurance.

BD you mention that AGM batteries of which I have 5 will accept a charge at a rate of 100% until they're full, unlike wet cells which charge at 100% until they're 80% full and then the charge rate falls off dramatically. Is this correct?

I agree that there seems to be no one correct way to solve this electrical issue. In talking to my wife see wants to replace the refrig until I told her how much one costs and it still doesn't fix the charging issue.

FF wrote:
Many small (under 10KW ) noisemakers will not power any large amperage battery charger to rated power,

-- Edited by FF on Thursday 1st of March 2012 03:28:12 PM

Click to expand...

*Glad you mentioned that. I had thought about that, but didn't really check it out.

If my calculations are correct my 7.5 KW genset should put out 62.5 amps at 120 volts. According to the specs on the 100 amp charger "the Input current at rated output (AC amps)" is 15.

I'm guessing I'm OK as long as I don't run the AC and microwave.

timjet wrote:JD wrote:
Timjet,

I'm still having a little problem with a statement that you made on page two. "My twin '98 Cummins diesels are due for a front end alignment, according to the guru's on boat diesel. Meaning everything on the front of the engine that spins, idler pulley, alternator, and supporting stuff should be replaced. ".

I'm not a Cummins tech but statements like that*by mechanics sound like money makers and not*facts.* That being said.* If the guru was Tony Atherns then I might give it some credence.*

BTW I think you are on to what has to be done and there is no "one only way" to attack your problem.



-- Edited by JD on Friday 2nd of March 2012 08:03:15 AM

Click to expand...

*JD, that's very perceptive of you to pick up on that. The guru was indeed Tony and I just talked to him yesterday about what needs to be done.

I was incorrect in saying everything needs to be replace. The idler pulley has had a history of problems where the bolt that attaches the pulley to its mount has worn and in some cases has allowed the pulley to come loose. If this happens at high rpm, it can do big damage to the engine. Tony has talked to Cummins about this and they have made some minor changes, but not to Tony's satisfaction. He sells an upgrade kit for these pulleys that costs I believe less than $50. The other issue is the belt tensioner. It is pretty reliable from what I believe Tony said, but based on the age of my engines, '98, he suggests taking it apart, cleaning and re-lube them. The alternator is the other thing he thinks I should replace, primarily because of the bearings. If the alternator freezes up it could cause damage. Tony is a pro-active guy as you probably know and he knows what fails on these engines. I think he's trying to give me an heads up so I can prevent future problems. Besides he has no real skin in the game, but I think I will probably just send the idler pulley and belt tensioner to him and let him refurb them for me. Cost I believe was less than $100 per engine. Pretty cheap insurance.

BD you mention that AGM batteries of which I have 5 will accept a charge at a rate of 100% until they're full, unlike wet cells which charge at 100% until they're 80% full and then the charge rate falls off dramatically. Is this correct?

I agree that there seems to be no one correct way to solve this electrical issue. In talking to my wife see wants to replace the refrig until I told her how much one costs and it still doesn't fix the charging issue.

Timjet,

*That sounds better and Tony is a person to be trusted from what I can tell.*Getting his beefed up kit is probably a good idea. I like everything he has said but I think I might check on an alternator shop in Tampa and get new bearings put into the ones you have and be done.* Truth be known I'm not sure I would do even*that but if you would feel better that is going to be a hell of a lot less money.* Now if you want to jump up the amperage on the alts that is a different story.

Your battery charging statement is incorrect.* AGM charge or accept 100% of the charge up to*a point near full charge then the voltage needs to be bumped*up*.2 V or something like that to get over the hump and get to a full charge at which time the charger or alt can go to a float.* A wet cell battery can only accept 50% of a charge rate and then it has the same problem at some point as the*AGM*and the regulator needs to increase the voltage a bit*until it is at full charge at which time it goes to a float as well.* So a 400 amp*bank of*AGMs at half charge, ie*200 amps needs four hours + on a 50 amp alt or*two hours+ on a 100 amp alt or one hour+ on a 200 amp alt or half an hour on a 400 amp alt..... Get the picture.* Wet cells can only by nature of their design accept 50% of that available charge so take the previous numbers and double the time of the charge for each alternator.

Here is a good read site for you.* It is about using batteries at your house to be off the grid but the information on the batteries is still good on a boat or what ever.

http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Voltages

I think once you read this a lot of this will become more clear.

BD, thanks for that web site. I'll sit down tonight and read through that, good stuff.

One of my battery banks is not accepting a charge. When I arrived at the boat the other day, the batteries were fully charged reading about 13.6 volts with the charger on. I turned the charger off and after 2 hours with no DC load, the No 2 bank read 12.6 volts and the No 1 bank read 13.2 volts. I'm going to disconnect the 2 batteries on the No 2 bank and test them individually by again charging them to full capacity and then testing after 2 hours off the charger and no load. The batteries are 2 years old from West Marine and if I determine one or both are bad I'll try to get pro-rated new ones. Is my thinking correct??

timjet wrote:
BD, thanks for that web site. I'll sit down tonight and read through that, good stuff.

One of my battery banks is not accepting a charge. When I arrived at the boat the other day, the batteries were fully charged reading about 13.6 volts with the charger on. I turned the charger off and after 2 hours with no DC load, the No 2 bank read 12.6 volts and the No 1 bank read 13.2 volts. I'm going to disconnect the 2 batteries on the No 2 bank and test them individually by again charging them to full capacity and then testing after 2 hours off the charger and no load. The batteries are 2 years old from West Marine and if I determine one or both are bad I'll try to get pro-rated new ones. Is my thinking correct??

Click to expand...

*How do you know there was no load.* Just because it doesn't read on your meter doesn't mean there was no load.*

With nothing on check if there is any current flowing by disconnecting at the battery and using a hand held meter between the battery and the B+ wire* or cable. Check that something isn't directly connected to the B+ of that bank and isn't going through the battery cable or connected the other B+ of the two battery set.* PO's do strange things. *Another way is to charge them to full 13.6V and then disconnect the ground on that set of batteries.* let them rest for a time and then see where they are before connecting them back up.* They may just surprise you.


-- Edited by JD on Friday 2nd of March 2012 11:08:54 AM

timjet wrote:
BD, thanks for that web site. I'll sit down tonight and read through that, good stuff.

One of my battery banks is not accepting a charge. When I arrived at the boat the other day, the batteries were fully charged reading about 13.6 volts with the charger on. I turned the charger off and after 2 hours with no DC load, the No 2 bank read 12.6 volts and the No 1 bank read 13.2 volts. I'm going to disconnect the 2 batteries on the No 2 bank and test them individually by again charging them to full capacity and then testing after 2 hours off the charger and no load. The batteries are 2 years old from West Marine and if I determine one or both are bad I'll try to get pro-rated new ones. Is my thinking correct??

Click to expand...

You might just be looking at the surface charge on the 13.2V bank.* Like JD says, there may be a small unknown load on the 12.6 bank.* Besides, 12.6 is a full charge.* Take a look at the specific gravities of the cells to confirm a weak cell.

If you don't have a battery monitor to watch loads and charges, and the banks are identical, and if you have the time and inclination, you could swap the batteries in the 2 banks which will allow you to clean up the terminals and tighten all connections in the process.* If the low voltage stays on the same bank, you have a persistent load and your bbatteries are fine.

JD wrote:*How do you know there was no load.* Just because it doesn't read on your meter doesn't mean there was no load.*

Click to expand...

*I don't have a DC amp meter. I had just turned off everthing DC. But you're right there could have been something powered and I was unaware of it. I'll disconnect the batteries after a full charge and check them again after 2 hours.

FlyWright wrote:

Click to expand...

You might just be looking at the surface charge on the 13.2V bank.* Like JD says, there may be a small unknown load on the 12.6 bank.* Besides, 12.6 is a full charge.* Take a look at the specific gravities of the cells to confirm a weak cell.

If you don't have a battery monitor to watch loads and charges, and the banks are identical, and if you have the time and inclination, you could swap the batteries in the 2 banks which will allow you to clean up the terminals and tighten all connections in the process.* If the low voltage stays on the same bank, you have a persistent load and your bbatteries are fine.

*It looks like they charged to 75%. I'll check again after disconnecting each battery from the bank.
<h3>SOC (%) vs. OCV
the battery State Of Charge vs battery's Open Circuit Voltage</h3>
An easy method to estimate the State of Charge (SOC) of the battery is by measuring its Open Circuit Voltage (OCV). This measurement should be made after the battery has been at rest for a minimum of four hours with the battery shut off from its charging source and load. The voltage is listed as Volts/cell and for a 12V (6 cell) battery at 25°C (77°F).


<center><table border="1" cellpadding="5"><tbody><tr><td>State of Charge (%)</td><td>OCV per cell</td><td>OCV per 12V battery</td></tr><tr><td>100</td><td>2.17 or greater</td><td>13.0 or greater</td></tr><tr><td>75</td><td>2.10</td><td>12.6</td></tr><tr><td>50</td><td>2.03</td><td>12.2</td></tr><tr><td>25</td><td>1.97</td><td>11.8</td></tr><tr><td>0</td><td>1.90 or less</td><td>11.4 or less</td></tr></tbody></table></center>

<h3>*</h3><h3>Lead-Acid State of Charge</h3><h4>Here are no-load typical voltages vs state of charge</h4>
(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.
<table style="width:400px;" border="1"><tbody><tr><th bgcolor="#ffffff" width="132">State of Charge</th><th bgcolor="#ffffff" width="132">12 Volt battery</th><th bgcolor="#ffffff" width="132">Volts per Cell</th></tr><tr><td width="132">100%</td><td width="132">12.7</td><td width="132">2.12</td></tr><tr><td width="132">90%</td><td width="132">12.5</td><td width="132">2.08</td></tr><tr><td bgcolor="#00ff00" width="132">80%</td><td bgcolor="#00ff00" width="132">12.42</td><td bgcolor="#00ff00" width="132">2.07</td></tr><tr><td bgcolor="#00ff00" width="132">70%</td><td bgcolor="#00ff00" width="132">12.32</td><td bgcolor="#00ff00" width="132">2.05</td></tr><tr><td bgcolor="#00ff00" width="132">60%</td><td bgcolor="#00ff00" width="132">12.20</td><td bgcolor="#00ff00" width="132">2.03</td></tr><tr><td bgcolor="#00ff00" width="132">50%</td><td bgcolor="#00ff00" width="132">12.06</td><td bgcolor="#00ff00" width="132">2.01</td></tr><tr><td bgcolor="#00ff00" width="132">40%</td><td bgcolor="#00ff00" width="132">11.9</td><td bgcolor="#00ff00" width="132">1.98</td></tr><tr><td bgcolor="#ffff00" width="132">30%</td><td bgcolor="#ffff00" width="132">11.75</td><td bgcolor="#ffff00" width="132">1.96</td></tr><tr><td bgcolor="#ffff00" width="132">20%</td><td bgcolor="#ffff00" width="132">11.58</td><td bgcolor="#ffff00" width="132">1.93</td></tr><tr><td bgcolor="#ff0000" width="132">10%</td><td bgcolor="#ff0000" width="132">11.31</td><td bgcolor="#ff0000" width="132">1.89</td></tr><tr><td bgcolor="#ff0000" width="132">0</td><td bgcolor="#ff0000" width="132">10.5</td><td bgcolor="#ff0000" width="132">1.75</td></tr></tbody></table>


-- Edited by FlyWright on Friday 2nd of March 2012 11:56:05 AM

FlyWright wrote:timjet wrote:
BD, thanks for that web site. I'll sit down tonight and read through that, good stuff.

One of my battery banks is not accepting a charge. When I arrived at the boat the other day, the batteries were fully charged reading about 13.6 volts with the charger on. I turned the charger off and after 2 hours with no DC load, the No 2 bank read 12.6 volts and the No 1 bank read 13.2 volts. I'm going to disconnect the 2 batteries on the No 2 bank and test them individually by again charging them to full capacity and then testing after 2 hours off the charger and no load. The batteries are 2 years old from West Marine and if I determine one or both are bad I'll try to get pro-rated new ones. Is my thinking correct??

Click to expand...

You might just be looking at the surface charge on the 13.2V bank.* Like JD says, there may be a small unknown load on the 12.6 bank.* Besides, 12.6 is a full charge.* Take a look at the specific gravities of the cells to confirm a weak cell.

If you don't have a battery monitor to watch loads and charges, and the banks are identical, and if you have the time and inclination, you could swap the batteries in the 2 banks which will allow you to clean up the terminals and tighten all connections in the process.* If the low voltage stays on the same bank, you have a persistent load and your bbatteries are fine.

*All good valid*suggestions*but he*may not be able to*check the specific gravity on the AGMs.* I don't think he can check individual cell voltage either.*

JD wrote:FlyWright wrote:timjet wrote:
BD, thanks for that web site. I'll sit down tonight and read through that, good stuff.

One of my battery banks is not accepting a charge. When I arrived at the boat the other day, the batteries were fully charged reading about 13.6 volts with the charger on. I turned the charger off and after 2 hours with no DC load, the No 2 bank read 12.6 volts and the No 1 bank read 13.2 volts. I'm going to disconnect the 2 batteries on the No 2 bank and test them individually by again charging them to full capacity and then testing after 2 hours off the charger and no load. The batteries are 2 years old from West Marine and if I determine one or both are bad I'll try to get pro-rated new ones. Is my thinking correct??

Click to expand...

You might just be looking at the surface charge on the 13.2V bank.* Like JD says, there may be a small unknown load on the 12.6 bank.* Besides, 12.6 is a full charge.* Take a look at the specific gravities of the cells to confirm a weak cell.

If you don't have a battery monitor to watch loads and charges, and the banks are identical, and if you have the time and inclination, you could swap the batteries in the 2 banks which will allow you to clean up the terminals and tighten all connections in the process.* If the low voltage stays on the same bank, you have a persistent load and your bbatteries are fine.

*All good valid*suggestions*but he*may not be able to*check the specific gravity on the AGMs.* I don't think he can check individual cell voltage either.*

Forgot they were AGMs.* This data is for lead acids.

FlyWright, are those numbers for wet cells or AGM's?

oops we crossed paths

Timjet: interested to know where you got to with your original question, which was about a possible upgrade to your genset's 'start' alternator. I have been down this same path and was told that my 13.5KW genset couldn't take much more than a 20-30A alternator (as installed): the pulleys, belts and whole engineering of that end of the genset is only engineered to support a small alternator spec'd with the sole purpose of maintaining the charge on the genset start battery. I was advised not to even consider upgrading to a 'small' 60A alternator. Much better to address the real issue...inadequate House bank charging capacity....in the tried & proven ways of external smart regulation; larger proppulsion engine alts; and charge from both directed to house bank.

But where did you get to?

FlyWright said:

<h3>*</h3><h3>Lead-Acid State of Charge</h3><h4>Here are no-load typical voltages vs state of charge</h4>
(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.
<table style="width:400px;" border="1"><tbody><tr><th bgcolor="#ffffff" width="132">State of Charge</th><th bgcolor="#ffffff" width="132">12 Volt battery</th><th bgcolor="#ffffff" width="132">Volts per Cell</th></tr><tr><td width="132">100%</td><td width="132">12.7</td><td width="132">2.12</td></tr><tr><td width="132">90%</td><td width="132">12.5</td><td width="132">2.08</td></tr><tr><td bgcolor="#00ff00" width="132">80%</td><td bgcolor="#00ff00" width="132">12.42</td><td bgcolor="#00ff00" width="132">2.07</td></tr><tr><td bgcolor="#00ff00" width="132">70%</td><td bgcolor="#00ff00" width="132">12.32</td><td bgcolor="#00ff00" width="132">2.05</td></tr><tr><td bgcolor="#00ff00" width="132">60%</td><td bgcolor="#00ff00" width="132">12.20</td><td bgcolor="#00ff00" width="132">2.03</td></tr><tr><td bgcolor="#00ff00" width="132">50%</td><td bgcolor="#00ff00" width="132">12.06</td><td bgcolor="#00ff00" width="132">2.01</td></tr><tr><td bgcolor="#00ff00" width="132">40%</td><td bgcolor="#00ff00" width="132">11.9</td><td bgcolor="#00ff00" width="132">1.98</td></tr><tr><td bgcolor="#ffff00" width="132">30%</td><td bgcolor="#ffff00" width="132">11.75</td><td bgcolor="#ffff00" width="132">1.96</td></tr><tr><td bgcolor="#ffff00" width="132">20%</td><td bgcolor="#ffff00" width="132">11.58</td><td bgcolor="#ffff00" width="132">1.93</td></tr><tr><td bgcolor="#ff0000" width="132">10%</td><td bgcolor="#ff0000" width="132">11.31</td><td bgcolor="#ff0000" width="132">1.89</td></tr><tr><td bgcolor="#ff0000" width="132">0</td><td bgcolor="#ff0000" width="132">10.5</td><td bgcolor="#ff0000" width="132">1.75</td></tr></tbody></table>


-- Edited by FlyWright on Friday 2nd of March 2012 11:56:05 AM

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Say What?

D you mention that AGM batteries of which I have 5 will accept a charge at a rate of 100% until they're full, unlike wet cells which charge at 100% until they're 80% full and then the charge rate falls off dramatically. Is this correct?

NOPE.