Basic battery question

[dhays]

I am considering combining my house bank with my thruster bank. I asked this a year ago, but it was lost amidst other issues.

House bank is about 780AH made up of relatively new 4 x 390AH 6v AGM batteries. The thruster bank is 2 Lifeline 8D batteries at least 4 years old that would have been 500AH when new.

I know that it is considered a “bad idea” to combine batteries of different ages and combine batteries that aren’t identical.

So, here is one idea. What affect would there be if I combined them this way. How would each battery bank be deplete? Same AH from each or equal percentage from each or what?


This would make one bank. It would be easier to measure the total AH use from the this bank.

 

[Insequent]

There is very little difference between your two alternative wiring scenarios. Basically it is just different voltage drop arising from the difference in length of wires connecting the two batteries versus connecting each bank to the busbar. Assuming the busbars are in the general vicinity of the batteries, you might be hard put to measure the difference.

How will the banks deplete? I'll wait for expert commentary, but what should happen is that both banks will keep the same voltage while discharging. Due to differences in age the banks will likely have different internal resistance, leading to slightly different current draw. That effect could be magnified as the SoC reduces. I would think that more Ah will be provided by the larger bank. I don't see why this would be particularly detrimental to either bank until one bank has aged to the point it has only a fraction of its original capacity. But I may well be overlooking some other important factors.

 

[Comodave]

I would not do it especially with the different battery types and the different ages of the batteries. But if you are going to do it, do it the second way. You want to charge and draw across the whole bank otherwise you are wasting the second battery as it won’t charge properly and the second battery will essentially be invisible to the load.

 

[dhays]

Thanks for the reply. I’m so amazingly ignorant about electricity that it astounds even me.

If it is true that the effect is about the same between the two configurations, the first would be a lot easier. The diagram is simply a representation. The accomplish the first, I just need to add about a 6” inch cable to bridge two existing positive busbars. One busbar is connected to the thruster bank and the other to the house bank. So that solution has the advantage of being cheap and easy.

The second one would allow accurate measurement of the AH usage from the existing amp meter that is connected to the house bank or SmartGuage that I could add. The downside to the second is that the interconnects between the two banks will actually be about 8’ long. Even with a 2/0 cable, that would be a measurable voltage drop I would think.

 

[Comodave]

No they are not about the same. Talk to a knowledgeable marine electrician. If you don’t draw and charge across the whole bank, you might as well not add the other batteries. They will not charge fully due to the internal resistance in the cables. 8’ is probably too long for 2/0 battery cable depending on the current. The forum has a lot of great info, but you need an electrician on site to configure the entire system not just the batteries.

 

[alfamike]

different design batteries

Do not combine AGM batteries with Flooded lead acid batteries in same circuit. That is a big No No.

The AGM batteries are a completely different construction & need different charging profiles to keep circuit from ruining them.

You should not combine AGM batteries with Deep Cycle flooded lead acid batteries in the same circuit.

You will have all kinds of issues as they have completely different charging acceptance, different internal resistance, different needs & are so different in design that it will shorten their life.

So I would reconsider this approach.

Good Luck.

Alfa Mike

 

[ranger58sb]

I am considering combining my house bank with my thruster bank. I asked this a year ago, but it was lost amidst other issues.

House bank is about 780AH made up of relatively new 4 x 390AH 6v AGM batteries. The thruster bank is 2 Lifeline 8D batteries at least 4 years old that would have been 500AH when new.

What are your trying to gain? Or improve? What's "broke?"

-Chris

 

[boomerang]

I would vote a no, don't combine your thruster with your house batteries,even if the batteries were of the same type & the same age. The voltage surge from using the thruster would be hard on some of the other equipment on the house circuit, namely your voltage-sensitive electronics & LED lighting

 

[CMS]

Dave,

If you are looking to parallel them, when using the thruster for more oomph, then an ML-ACR, with some very large cable, will be the best option as it can be manually locked to on when thrusting so you can now take advantage of the alternator & house bank to better keep voltage up.

Doing this allows for a much smaller bow bank. You can then move that dead-lead from up in the bow and put that lead into the house bank on the next upgrade.

Batteries with similar or the same charging voltages can be paralleled for charging but you don't want different batteries, age, type, brand in parallel for discharging.

 

[O C Diver]

My bow thruster was originally tied to the house bank. It caused all kinds of problems from low voltage spikes when I pulsed the bow thruster. Converting the thruster to 24 volts and having the batteries within 2' of the thruster solved all the problems. If you need more house battery capacity, build a bigger house bank with all new batteries of the same type.

Ted

 

[dhays]

Dave,

If you are looking to parallel them, when using the thruster for more oomph, then an ML-ACR, with some very large cable, will be the best option as it can be manually locked to on when thrusting so you can now take advantage of the alternator & house bank to better keep voltage up.

Doing this allows for a much smaller bow bank. You can then move that dead-lead from up in the bow and put that lead into the house bank on the next upgrade.

Batteries with similar or the same charging voltages can be paralleled for charging but you don't want different batteries, age, type, brand in parallel for discharging.

Thanks Rod.

My idea was to simply create a larger house bank and not have that 500AH of batteries just sitting there doing nothing most of the time. I have plenty of power for the thrusters and windlass with the existing setup.

My real concern was the effect of combining the newer AGM deep cycle batteries with the older Lifeline AGM during discharge. So, it sounds like I’ll give up on the idea for now.

Hopefully, the Lifeline’s will perform for many years to come. They are really pampered. While I use my thrusters frequently, it is only in extremely short bursts when arriving or leaving the dock. The bank also powers the windlass, but that is a pretty easy job as well. The rest of the time they are kept charged up by a Balmar Duo-charger set to the Lifeline charging profile.

If they fail, I may revisit the idea and possibly replace them with a couple of the same battery that I used in my house bank, and then parallel them into a single bank.

Out of curiosity, is there much of a difference for discharging in the two proposed diagrams I listed?

 

[ranger58sb]

My idea was to simply create a larger house bank and not have that 500AH of batteries just sitting there doing nothing most of the time. I have plenty of power for the thrusters and windlass with the existing setup.

What powers your electronics? House bank? If so, and if you combine the two banks, you might find that voltage drop caused by heavy current draw from the thruster could cause some of your electronics components to crap out momentarily.

From reading, it seems the best thruster installations have those batteries as nearby as possible... and it sounds like you've got that?

-Chris

 

[Insequent]

Although I would not combine as Dave is contemplating, 'conventional wisdom' and all that, I had hoped someone could provide some science/physics as to why not.

The high-draw voltage drop story I don't buy, at least for short periods. My inverter can draw 275A from the house bank without impacting my electronics. The voltage drops over time, it does not spike, and the inverter loads are not on long enough to flatten the house bank completely when there would be a problem for other stuff.

I can see that different age banks (but same battery type, as Dave has) could be an issue charging, as the newest bank may not be able to get to 100%. But discharging? Any science for not doing it? Is it a case of 'you can get away with it for a while, but in the longer term you reduce the number of discharge cycles before the bank craps out'.

Also, note for CMS: Dave I believe has both banks in lazarette. No bank in bow. And stern thruster as well as bow thruster.

 

[dhays]

Although I would not combine as Dave is contemplating, 'conventional wisdom' and all that, I had hoped someone could provide some science/physics as to why not.

The high-draw voltage drop story I don't buy, at least for short periods. My inverter can draw 275A from the house bank without impacting my electronics. The voltage drops over time, it does not spike, and the inverter loads are not on long enough to flatten the house bank completely when there would be a problem for other stuff.

I can see that different age banks (but same battery type, as Dave has) could be an issue charging, as the newest bank may not be able to get to 100%. But discharging? Any science for not doing it? Is it a case of 'you can get away with it for a while, but in the longer term you reduce the number of discharge cycles before the bank craps out'.

Also, note for CMS: Dave I believe has both banks in lazarette. No bank in bow. And stern thruster as well as bow thruster.

I’ve looked at the charging issue, and both lifeline and US batteries give voltage ranges for their charge profiles. The ranges on the two types of batteries are not the same, but they overlap. For example, the US Batteries want an Absorb voltage of 14.4-15.0v. Lifeline 14.2-14.4.

My concern was the effect of discharging two different types of batteries of two different ages. I am happy to believe CMS when he tells me it is bad idea but I do share your curiosity as to exactly why it could pose problems on the discharge side. My guess (remember I know nothing) is that one bank would tend to discharge faster than the other due to inevitable differences in internal resistance.

You are correct that all my batteries are in the aft lazarette, about 4 feet apart. The thruster bank controls bow and stern thrusters as well as the windlass.

 

[alfamike]

Different Type Batteries wired into parallel

As I understand it, We are comparing a thin plate starting type AGM battery to a thick plate deep cycle AGM battery. these are two different battery designs. these do have different operating characteristics & specs for recharging.
Also - BTW - AGM's as a rule of thumb should not be charged over 14.4 to 14.5 volts or they gas to much & will pop the sealed gas escape valves.
They are also not supposed to be always on a float voltage like you can do on a flooded lead acid battery, as they will over heat.


The scientific reason that the two different designed batteries should not be combined is that they are different designs & both charge & discharge differently by design. They are not designed to work together in a parallel circuit. They will always be at odds with each other in OCV.


That said, they can be used in a serial circuit.

If they are wired in parallel this may make an in circuit voltage difference in the batteries that are hooked up in parallel between the different design batteries in that circuit during different periods of operation.

So there can be times even after you turn off both the external load or charging sources, the batteries are then still working & trying to equalize the voltages between them to be the same in an unregulated manor.
This means that one battery set discharges either forward or backwards into the other batteries at an uncontrolled rate while it is just sitting there. You can't see the electricity moving, but it is still happening

Obviously you are doing what ever you want to do as it is your boat. The issues are not there every second that they are hooked together. However, they can occur at unpredictable times.

However, remember these batteries are both filled with sulfuric acid, make flammable hydrogen gas & are very powerful, can go into thermal runaway & are not "D" cells in a flash light & their can be consequences which can possibly occur & we are just trying to warn you to not make this mistake.

Good luck.

Alfa Mike

 

[boomerang]

Although I would not combine as Dave is contemplating, 'conventional wisdom' and all that, I had hoped someone could provide some science/physics as to why not.

The high-draw voltage drop story I don't buy, at least for short periods. My inverter can draw 275A from the house bank without impacting my electronics. The voltage drops over time, it does not spike, and the inverter loads are not on long enough to flatten the house bank completely when there would be a problem for other stuff.

I can see that different age banks (but same battery type, as Dave has) could be an issue charging, as the newest bank may not be able to get to 100%. But discharging? Any science for not doing it? Is it a case of 'you can get away with it for a while, but in the longer term you reduce the number of discharge cycles before the bank craps out'.

Also, note for CMS: Dave I believe has both banks in lazarette. No bank in bow. And stern thruster as well as bow thruster.

"The high-draw voltage drop story I don't buy, at least for short periods. My inverter can draw 275A from the house bank without impacting my electronics. "
Apples & oranges. Your inverter "can" draw 275Amps. I doubt you're surging that kind of a load from your inverter all at once or continuously . Even a small thruster WILL be using 275 Amps or more and quite often.

 

[Insequent]

Apples & oranges. Your inverter "can" draw 275Amps. I doubt you're surging that kind of a load from your inverter all at once or continuously . Even a small thruster WILL be using 275 Amps or more and quite often.

I have drawn high amp from the inverter, and occasionally overload it. The clothes dryer for example draws a lot. And my thruster/windlass draws do not get above 150A, as displayed on the battery monitor for that bank in my bow.

Edit: apologies, just tested my thruster and it draws 338A.

 

[Insequent]

The scientific reason that the two different designed batteries should not be combined is that they are different designs & both charge & discharge differently by design. They are not designed to work together in a parallel circuit. They will always be at odds with each other in OCV.

..snip

Alfa Mike

I understand both banks are AGM

 

[boomerang]

I have drawn high amp from the inverter, and occasionally overload it. The clothes dryer for example draws a lot. And my thruster/windlass draws do not get above 150A, as displayed on the battery monitor for that bank in my bow.

You run your clothes dryer on your inverter???!!!
I think I know the source of your occasional inverter overload...

 

[Insequent]

You run you clothes dryer on your inverter???!!!
I think I know the source of your occasional inverter overload...

I usually run the clothes dryer only when underway. But occasionally I start it before actually getting underway. And yes, I know why the inverter overloads, if it does!

Back to urban myths etc relating to batteries.....

 

[Baker]

I also have heard that when it comes to AGM, there is no such thing as a "start" battery and "deep cycle" like there is for flooded cell batteries.

 

[dhays]

Based on CMS’s input earlier, I have given up on the idea. I was curious as to the reasons thought.

 

[ranger58sb]

I also have heard that when it comes to AGM, there is no such thing as a "start" battery and "deep cycle" like there is for flooded cell batteries.

FWIW, Lifeline makes a distinction, details on their website.

-Chris

 

[ranger58sb]

The high-draw voltage drop story I don't buy, at least for short periods.

I've had that happen when re-starting an engine (same dual-purpose bank that services our electronics) after trolling on the other engine for 4-5 hours.

In that kind of case, that meant the "electronics bank" had been drawn down quite a bit over the course of those 4-5 hours... but I don't know exactly how much.

I've assumed a thruster can be calling for similar amps (or maybe more?) as our engine's starter motor.

OTOH, I may well have mis-diagnosed the event, and it could well have been something about the way our electronics installer guys tied into various circuitry on the boat... given that any re-start of that engine also restarts all our electronics. IOW, I've just assumed it was all about voltage drop...

-Chris

 

[FF]

"given that any re-start of that engine also restarts all our electronics"

In many vehicles the engine start (frequently a key switch) will cut off all the accessories .

This is proper and well engineered precaution, as should the starter motor fail to release , it will become a generator.

This can pump over 100V into any circuit that is connected.

Much ungood for most bulbs and electronic devices.

 

[Insequent]

"given that any re-start of that engine also restarts all our electronics"

In many vehicles the engine start (frequently a key switch) will cut off all the accessories .

This is proper and well engineered precaution, as should the starter motor fail to release , it will become a generator.

This can pump over 100V into any circuit that is connected.

Much ungood for most bulbs and electronic devices.

A good boat system is superior to that of a motor vehicle. eg Dedicated start battery isolated from house loads when engines are not running, to ensure that the start battery can actually perform as required. And only paralleled after engines have been started, partly for reasons FF describes.

Yes, there are boats that use the same battery/bank for house/starting. But that is not a good boat system at all. A battery configuration that cuts power to electronics for engine starting is a consequence of a poorly designed/installed system and a red flag for me.

 

[dhays]

Yes, there are boats that use the same battery/bank for house/starting. But that is not a good boat system at all. A battery configuration that cuts power to electronics for engine starting is a consequence of a poorly designed/installed system and a red flag for me.

Most sailboats that I’ve seen are supplied by the factory with a combined start/house bank. Many use a split system (ie a 1, 2, Both switch) that simply controls two equally sized banks. Some sailors will add a separate start battery to avoid the risk of a battery that is drained to the point that they can’t start the engine and to enable the use of both halves of a split system.

So while it may not be considered a good system, it is a system that is used a whole lot of FD hulls with small Diesel engines.

 

[ranger58sb]

Yes, there are boats that use the same battery/bank for house/starting. But that is not a good boat system at all. A battery configuration that cuts power to electronics for engine starting is a consequence of a poorly designed/installed system and a red flag for me.

I wouldn't have chosen to design our system that way, either, but it's apparently quite common and actually has an advantage or two... but all that aside, it is what it is, and the prices were in keeping with deign features.

I could change it, with significant effort and expense, without much gain.

-Chris

 

[JDCAVE]

Dave: my house bank was originally configured similar to the first figure in your original post. This was because the battery boxes were not close together so they were combined at the + and - buss bars. The problem was the alternator was connected to one of the banks and the other bank was being charged via the buss bar, with a likely voltage drop. I later moved the 2nd battery box and cables the batteries according to figure 2 in your original post, to better balance charge and discharge across the batteries.

The first figure will work but isn’t “as optimal” as it could be, or so I’m told. But because you have the thruster and house banks separate, it might be a more viable option. That said, the biggest issues that have been alluded to remain: age of batteries and different types.

Jim