% of battery charge by voltage

[Phil Fill]

On one of the discussion I mentioned the % of the battery charge can be estimated by the voltage of the battery. The battery has to be at rest, not charging, and no high usage. The amp reading on the Eagle is normal 1 amps just some lights on, which I call rest. When the battery get down around 12.5 time to charge. Been using this method for 40+ years. I believed most expensive fancy battery monitors are just reading the volt?



13.2+ volts – 100%
12.8 volts – 75%
12.4 volts - 50%
12.2 volts – 25%
11.8 volts – dead

The % is also on some volt meters. Anyway it's close enough for me.

 

[Larry M]

On one of the discussion I mentioned the % of the battery charge can be estimated by the voltage of the battery. The battery has to be at rest, not charging, and no high usage. The amp reading on the Eagle is normal 1 amps just some lights on, which I call rest. When the battery get down around 12.5 time to charge. Been using this method for 40+ years. I believed most expensive fancy battery monitors are just reading the volt?



13.2+ volts – 100%
12.8 volts – 75%
12.4 volts - 50%
12.2 volts – 25%
11.8 volts – dead

The % is also on some volt meters. Anyway it's close enough for me.

What kind of batteries do you have? Our wet cells are full at ~12.70 volts and 50% full the voltage reads ~12.20.

AGMs and Gels are full at or just under 13.0 volts.

 

[DavidM]

Go to http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf and read Trojan's User Guide for batteries. It says that measuring voltage is the least prefered way to measure state of charge. But in any case you must wait 6 hours with no in or out current to get a true reading.

On one of the forums, someone posted voltage readings vs time for a fully charged battery after the charging source was removed. It took as much as 24 hours for the voltage to stabilize. But after 6 hours as recommended by Trojan, the voltage was pretty close to its final measurement.

Table 7 in the Trojan document gives the resting voltage vs state of charge for flooded cell batteries. 12.7 is 100%, 12.1 is 50% and 11.5 is 10%.

David

 

[RT Firefly]

Greetings,
How about one of those battery load testers? Any wait time?

 

[Phil Fill]

What kind of batteries do you have? Our wet cells are full at ~12.70 volts and 50% full the voltage reads ~12.20.

AGMs and Gels are full at or just under 13.0 volts.

Wet! I said this was an estimate and what is on some volt meters. If you use a different % that is up to you. I use 12.5 is when to charge. If you want to use 12.2 great! I do not want to get into a contest over the %.

MY point is/was the % charge can be estimated by voltage and fance expensive monitoring is not really necessary.

 

[FlyWright]

I agree, Phil. I started years ago with a similar chart. Even though I now have a battery monitor, I still keep the chart handy for reference, but never refer to it.

 

[motion30]

The voltage values seem high to me

 

[Aquabelle]

The 'fancy' battery monitors don't use voltage...they count amps in and out and are highly accurate. When a battery manufacturer like Trojan tells you not to use/rely on the voltage 'method', it would be wise to take note.

 

[psneeld]

Trying to use voltage is a real crapshoot depending on how "scientific" you want to be by waiting without ANY load on your batteries (not real practical for a cruiser)....will it give a rough idea??? Sure but how "rough" is good enoug...you might be better off "guessing" based on knowing what loads have been on for how long.

RT...the battery tester shown is a great tool for testing the cranking amps of a battery...a little less useful in telling the health of a deep cycle and darn near useless telling how many amp hours are left in a battery bank.

 

[bobofthenorth]

Each to his own but to the OP's question about whether voltage is what the "fancy" meters use, I think that has already been answered with a firm "NO". I think I paid just over $200 for my Trimetrics (one on the boat and one in the bus) - best $200 I ever spent. "Resting voltage" doesn't mean you turn off the loads and take a reading - it means the batteries have to sit for a while before you take the reading - that's simply not practical for most real world use.

 

[psneeld]

Each to his own but to the OP's question about whether voltage is what the "fancy" meters use, I think that has already been answered with a firm "NO". I think I paid just over $200 for my Trimetrics (one on the boat and one in the bus) - best $200 I ever spent. "Resting voltage" doesn't mean you turn off the loads and take a reading - it means the batteries have to sit for a while before you take the reading - that's simply not practical for most real world use.

One of many expert/battery site recommendations....

Open-Circuit Voltage Test:
You will need a Voltage Meter to perform this test (D.C. side)
For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.
1. Disconnect all loads from the batteries.,,,etc....etc


Not in all the reading I have done....that's why it's not practical for "in use" batteries....

that's why I mentioned using this method is a crapshoot...but do admit that once you know your battery bank you may have a reasonable idea when it needs charging.

 

[FF]

"The 'fancy' battery monitors don't use voltage...they count amps in and out and are highly accurate. When a battery manufacturer like Trojan tells you not to use/rely on the voltage 'method', it would be wise to take note."

In addition the unit "learns" just what it takes to fill and discharge your specific batt bank.

This is very useful as the batt set ages over the years and the banks capacity shrinks.

 

[bobofthenorth]

In addition the unit "learns" just what it takes to fill and discharge your specific batt bank.

I'm not exactly sure what you mean by that but I don't think that is true. What does happen is that owning a SOC meter forces you to better understand how your battery bank charges and specifically what constitutes a "100%" charge. Its not quite as simple as hooking up the meter and watching the needle move up or down. Measuring watts is a little trickier than measuring water or fuel. It is however orders of magnitude better than trying to use volts to measure battery SOC.

 

[Phil Fill]

I did say the voltage is an estimate and did not imply it was the best. However, it does tell me to start charging the batteries. What I look at is the color zone the neddle is in, green 100%; yellow 75%, and red 50% time to charge. If I was concerned I would spend the money on a bigger battery bank and/or charger before monitors, which is what I have done. Anyway, works for me.

We have three 8D batteries, 600+ amps, 300 usable amps that lasts 8 hous to get us though the night with temps below freezing. We have gone with out power for weeks at a time and the voltage % works for me.

 

[sunchaser]

Google Magnum BMK for a good description of their shunt (and related parts and pieces) and how it allows measurement of % charge and real time amps, volts, ah in/out and min/max DC volts.

If your boat is a dock fixture there is probably not a lot to be gained with a BMK, but if you cruise and anchor out a lot they are pretty informative. The BMK type devices biggest customer bases are motor homes, standby gensets/battery banks and hybrid/electric cars.

 

[rwidman]

Pretty much the only way to accurately determine the state of charge of flooded cell batteries:

Of course, it's pretty much a PITA to use and can't be used on sealed batteries.

 

[sunchaser]

Ron

Right you are, along with the dial phone and tube & CRT TVs, the SG float devices have their following.

Years ago the favorite trick for a few fly-by-night gas stations was to pull an electrolyte sample with a "specially calibrated" SG device and say "your battery is shot."

 

[rwidman]

Ron

Right you are, along with the dial phone and tube & CRT TVs, the SG float devices have their following.

Here's my phone:

 

[BruceK]

I did say the voltage is an estimate and did not imply it was the best. However, it does tell me to start charging the batteries. What I look at is the color zone the neddle is in, green 100%; yellow 75%, and red 50% time to charge. If I was concerned I would spend the money on a bigger battery bank and/or charger before monitors, which is what I have done. Anyway, works for me.

And me. My voltmeter is a useful guide, by no means "state of the art". You know your boat and what you expect it to read in most circumstances if all is well.
The hydrometer is useful testing a battery suspected to be on the way out, for defective cell(s) etc. A cheap accurate test to save junking a battery if it proves not to be the real culprit.

 

[caltexflanc]

Actually, the correct answer is "all of the above", but especially the hydrometer. We have a very inverter-intensive cruising lifestyle, preferring to anchor out or take a mooring (we used to live full time for months on a mooring). We have had a full Magnum system on board for about 5 years, Ms 4024, BMK and AGS.

The issue with the BMKs, as nice as they are, is twofold: 1) battery manufacturers tend to lie about the amp hour capacity of deep cycle batteries. Magnum wisely advises you to round down when inputting your capacity. 2) As a bank ages, capacity reduces. Towards the end of its life, our most recent bank, originally 400+amp hours at 24 volts, realistically had a capacity of maybe 100 amp hours. I had to keep dialing the AH down in the BMK, based on hydrometer and voltage readings, and then triangulate from there.

The only way to know true state of charge is via hydrometer. Of course, you don't have this option with AGMs or gels. So as the bank gets old, you have to look at volts as well.

For true deep cycle batteries, open circuit 12.5 volts is way high for "need" to charge. Thats 80% SOC on a true deep cycle. 12.1 (1.17SG)or 12.2(1,2SG), i.e. 50 or 60% is more like it.

 

[FF]

"2) As a bank ages, capacity reduces."

One usual reason for this is the failure to recharge to 100% full each and every discharge.

As the last 10% takes "forever" most noisemaker folks wont bother.

Only solar ( wind is too loud ) seems to be able to get top 100% with great regularity.

The system should be sized so the boat can live between 50% and 85% SOC , and allow for the system shrinkage that is normal with only 85% recharges.

 

[sunchaser]

My last set of Trojan 105s (house) went for 8 years with about 90% effective capacity still remaining, so said the Link monitor. Like tires, I replace batteries early but had I tried could have probably got another year or two out or them.

For several years I sold lead to battery manufacturers and learned about wet acid battery construction, some companies do it better than others. A quick read of Consumer Reports says the same, Die Hard always seems to top the list for cars. As stated by manufacturers, long battery life is normally related to using better marine rated charging systems, keeping them above 50% rated capacity and re-charging fully as says FF. For house, I've used Trojan brand for the past 30 years and never been let down. My industrial experience showed using NAPA with a 48 month guarantee was the most effective solution for large diesel engine starting.

If you enjoy gassing every year or so, you may add another few years of useful life, to your house batteries that is. This subject can become highly charged. Thanks P/F for starting it.

 

[rwidman]

................... As a bank ages, capacity reduces. ...........

Correct. And that's why. unlike a fuel tank, you cannot determine the remaining power by calculating power used against power added (by charging).

Another factor is the recommendation to use no more than 50% of the capacity, then recharge. Even ignoring that recommendation, unlike the fuel system where the last gallon of fuel is as good as the first, as the battery nears discharge, the voltage lowers to the point where many loads won't work properly or at all.

 

[sunchaser]

But Ron, ultimate battery life is up to the abuser/user. Waving a hydrometer around is like using a thermometer for deducing if you have a hangover. Like many things in life, treat batteries right and they go for a long time. Just like diesels.

Another way to look at it is why does a routinely waxed boat look better and have higher resale value longer than its unwaxed counterpart - because the owner cares. The list of care vs life goes on and on, indeed including lead, acid and allowable voltage fluctuation.

 

[Phil Fill]

Sq





I started this disvussion to give a cheap siple way of estimating % of charge left. Not to start a long discussion. I check the batteries omce a month. The batteries last an average of 7 years. Use it. Dont use it. Its up to you.

 

[sunchaser]

P/F

Of course you know there is no such thing as a short discussion on TF regarding something 100% of us have buried in our hulls.

 

[psneeld]

Sq





I started this disvussion to give a cheap siple way of estimating % of charge left. Not to start a long discussion. I check the batteries omce a month. The batteries last an average of 7 years. Use it. Dont use it. Its up to you.

The reason it turned into a discussion is there is disagreement whether your method is "good enough".

 

[BruceK]

Only solar ( wind is too loud ) seems to be able to get top 100% with great regularity.

90 watts of solar to each (of 2) 200ah wet cell batteries keeps them full and pretty much supplies day fridge use. 90watts x2 does much more than maintain, 25watts should do that, it does for the genset 150ah battery. Panels are getting cheaper, your first cost of installing is your last cost, but do fit a regulator (90w ones have equalization, but not the 25w panel).
Your batteries will appreciate the charge, top them up at least monthly.

 

[Andy G]

Sq





I started this disvussion to give a cheap siple way of estimating % of charge left. Not to start a long discussion. I check the batteries omce a month. The batteries last an average of 7 years. Use it. Dont use it. Its up to you.

Phil, staring a thread is a bit like having kids, your create them but where they end up is out of your hands.

 

[Edistoart]

If I may make a contribution to this discussion, I am an electronic technician and some years ago I did some research into the discharge/charge performance of gel lead acid batteries for my employer. My more useful and recent experience is with a 105 amp hour deep cycle battery I bought from K Mart to run a trolling motor on my canoe. I believe its draw was about 35 amps wide open. When underway I kept a small quality digital multimeter (3 1/2 digits) connected across the battery so I could watch the voltage fall. Running the motor at a little less than full power I got 4 or 5 hours cruising before the voltage fell to 11.1 to 11.5 volts. This is clearly more than the recommended discharge to 50% charge level (probably down to 10-20%) and the motor was running noticeably slower.

Rules for maximum life span (number of charge cycles) for a lead acid battery:
1. Always keep the battery fully charged.

2. Always keep the water level above the top of the plates.

Obviously Rule 1 isn't practical for yacht house banks; they are going to sit partially discharged for one or more days before recharging. This problem will be solved when the price of lithium ion batteries becomes reasonable and to a lesser extent lithium polymer batteries. LI batteries couldn't care less if you discharge them to 0%. In fact I have read that if you are going to store them for any length of time they should be discharged fully first! Talk about the very opposite from lead acid. The problem here with lead acid batteries is that as they are discharged a layer of lead sulfate forms on one of the two battery plates. It is normally removed by the charging current but the longer you wait to recharge the harder the sulfate gets and in a short time it isn't removeable by normal charging voltages. Intelligent battery chargers' equalization function applies 16-16.5 volts to blast the sulfate loose or cause it to dissolve in the electrolyte. Sounds brutal to me!

I dealt with rule one by putting the battery on charge as soon as I got home. I used a simple minded $25 Auto store 10 amp charger which I left connected all night.

The fully charged voltage of a lead acid battery is 12.70 volts. I seldom if ever saw more than 12.69 volts. The battery has to be open circuit-no connections, or battery switch off, and it must have rested about 24 hours to remove the surface charge on the plates that forms during recharge- this is a layer of hydrogen bubbles that cover the plates. This surface charge is why you must apply higher voltages to recharge the battery- 13.7 volts in the beginning, rising to 14.4. Until this surface charge is removed the voltmeter reading is of no use.

But! You don't have to wait 24 hours! A small discharge current for a minute or two should do the trick. I have read several times that if you have been driving your car for awhile and want to check the battery voltage, just shut off the engine and turn on the headlights (about a 20 amp load) for 1-2 minutes, then turn them off. Then you should get a voltage of 12.70 volts or less if the battery has been fully recharged. Watch the voltmeter for 15-20 seconds to see if it rises. If it does, wait until it stops rising and you have your true open circuit rest voltage. It worked for me.

If you had an accurate table of terminal open circuit rest voltages versus %discharge you should be able to accurately determine the state of charge. The reason that intelligent chargers can't perform this simple test is that during charging, they have no quick way to remove the surface charge before measuring.

I apologize if this is longwinded but battery chemistry is not so simple and there is a lot of misinformation flying around,