A simple battery question

Well, yes and no. 24V is a nominal voltage but 25.6V is also a nominal voltage. Lead chemistries have a nominal voltage of 2V per cell, that's where the classic 6, 12, 24, 36, 48V circuits names come from.
Lithium chemistries have other nominal voltages, for lithium phosphate that's 3.2V per cell. Therefore a four cell battery will be nominally 12.8V and 8 cells is 25.6V.
Lithium ion has a nominal voltage of 3.6V/cell (most manufacturers) and lithium titanate 2.4V/cell.

We always need to refer to the chemistry type, otherwise we confuse things as above, for example the charging voltage for lithium phosphate is 3.6 or 3.65V wich is the same as the nominal voltage of lithium ion.

 

My advice is forget about nominal voltages and focus on real voltages because they are really important .

Standard alternator probably will never charge Lifepo4 to 100% , but that chemistry allow to use batteries in automotive/marine just plug and play , like lead acid chemistry .

I doubt that e.g mx/enduro Lifepo4 batteries utilize any BMS .

Couple words from one guy , long term cruising with family in Carribean : "We use lead acid only in our powerbank .Very dependable , very available" and he's a pilot and sometimes make money there , repairing electronics .

 

I've checked enduro batteries and actually they do utilize BMS.

Anyhow friend of mine bought big Lifepo4 battery for 4x4 , like Fallguy linked above , and after one year the battery is dead . Reason unknown .

I'm not battery guy , but I want to have controll on every cell of modern batteries . External advanced BMS and access to single cell .

 

Splitting hairs here but a fully charged, chemically balanced, cell, lead acid,

How many volts are in each cell of a 12 volt liquid filled battery?
A battery is made up of a number of cells, and the lead acid chemistry dictates a fully charged voltage of about 2.12 volts per cell. Thus, a nominal 6 volt battery has three cells with a full charge voltage of 6.3 to 6.4 volts, and a 12 volt battery has six cells, and a full charge voltage of 12.7 volts.


Battery University Homepage https://batteryuniversity.com/#google_vignette
This website offers a comprehensive discussion on all types of batteries, charging issues
It may contain an answer to the OP's question

 

The two cables don’t need to have exactly the same resistance in any meaningful sense. What matters more is cable size, connection quality, fuse layout, and whether one battery is being hit harder than the other because of how the bank is wired. If you keep losing the same battery, I’d be looking first at imbalance, uneven current sharing, parasitic loads tied to one battery, or a battery/BMS issue rather than small cable resistance differences. A wiring diagram or photo would help a lot.

 

I already answered the question. The engineer told me the batteries needed periodic equalizing.

In a series setup, what can happen is one battery can be charged to 12.3v and the other 12.1, then the next time charging at 24.4v they will charge at the prior ratio of not 50/50 and over 20 charge cycles; the one battery starts to get overcharged…

 

Now I'm left wondering if the individual cells inside each battery also need periodic equalizing. Or does each individual battery's BMS programing do that periodically? The individual cells inside a battery are sometimes selected for the closest matching possible when the battery is made, but they are not clones, AFAIK..

 

I do not like to be a contrarian but hooking 12 volt two batteries in series creating a 24 volt max ouput and pulling power from between them is not an issue.
Toyota did it in their Landcruisers back in the 70's, ie hooked up 12v lighting and internal vehicle circuits. We were installing 12 volt winches into these vehicles and contacted Toyota Tech directly and they said that is fine. (a winch could draw up to 300 or more amps at full load)

A qualification: There are 6 2.1 volt cells connected in series in a 12 volt battery creating a chemically and electrically balanced equation at 12.6 volts.

Series
2 12 volt battery in series and one is drawn down to 11 volts. When a 24 volt draw is created by closing any circuit involving both batteries, the 12 volt charged battery will attempt to
charge the 11 volt battery. An issue is that with the closed circuit the 12 volt battery will pump a lot of amps into the 11 volt battery and can create heat.
Any 24 volt charger will have a specific charge voltage so I do not understand why one batter should "start to get overcharged" The new smart charger do run a higher voltage on the charge
cycle when bringing up a battery from a discharge but will drop this voltage as it nears the 12.6 fully charged value.

Parallel
As the ground and positive is common to each other, two batteries are always in a closed circuit state and will always equalize. "always" assuming that the batteries can hold a full charge.
The problem is if one battery is heavily sulfated, ie unable to charge, then the higher voltage battery will continue to supply amps to the lower charged battery and this will create heat.

The 6 cells in a 12 volt battery will equalize when ever a circuit is closed.

 

Yes, the voltage might read the same under load, but may not be identical, open circuit. More importantly, the AH capacity of each individual cell will not be equalized or identical, is my understanding. Eventually the weakest cell may be driven to reverse charge with age or under deep discharge, as a possible failure mode. An equalizing float or overcharge sometimes helped to close the capacity gap with lead and nickel systems, but the stronger cells with slightly higher voltages would sometime foil the procedure if they were weakened by overcharge; also the strong might keep the low capacity cells from going to full voltage/ capacity. Years ago, batteries bolted with individual cells could be rebuilt by removing and replacing the weak or reversed cells, depending on the overall condition of the battery. I think the early solar power homes that used huge individual lead acid cells bolted together could be done that way. Also the Prius cars using NiMH could be rebuilt rather than replacing the entire battery pack, as I recall..

Ps. Unlike lead or nickel systems, I think at least some if not all Li systems incorporate an individual BMS for each individual cell within a battery. Might be necessary because of extreme fire hazards associated with Li, each cell has to be individually monitored.

 

I asked ChatGPT and chat said if you draw 12v from one of the two 12v batteries in series; then the drawn from battery cannot handle the incoming current as well when charging occurs.

Here are some estimates.

One battery at 10 one at 12.8.

Charge at 25.6v. Vdiff is 25.6-22.8 or 2.2V.

Typ resistance is maybe 0.05 Ohms.

Current rushes into weak battery at 44A. For many batteries; this charge rate is too high.

I think I probably misunderstood the engineer and assumed the overcharge went the other way without evaluating what really occurs well.

Appreciate the pushback.

 

Of course, if you deeply discharge the 12V draw; then the current flowing through the weakened battery can be even higher. I suppose there needs to be a calc done for total watts from the charger as that is a limit, but I think it is likely a problem and I don’t wish to do the calc at this hour.

 

Thanks, fallguy!
I believe the current has to be the same through both batteries when they are in series and being charged for Ni and Pb systems. The battery with the higher voltage will limit what the current will be for both batteries, and it will be the same for both. The battery at 12.8v will have a lower limit as to the amount of current that can be pushed through, and that same current is what will be pushed through the 10v one. When the 12.8v reaches full charge, say 14.4v it will only allow a trickle of current to go through if any. So if the max charge voltage is 25.6v, and the weaker battery has reached 11.2v or more, no further current can be pushed through, and that leaves the lower 10v one undercharged @ 11.2v. Calculation: 25.6-14.4-11.2 = 0 volts to push the charger current through?
Li systems are extremely sensitive to overcharge/discharge rate, and have strict parameters for voltage, temperature, max current in or out, etc which are monitored by the BMS which ideally shuts things down when there is a hazard. So it is more complicated for Li because of the limitations/ resets imposed by the BMS . Anyway that's the way I understand it, FWIW..

Ps. Thinking about it some more, I don't see how a low battery in series could be charged by the stronger one, seems like they would have to be in parallel for the strong one to dump charge into the weaker one? Also when two unmatched batteries in series are supplying power, the weaker one limits the current at the intended voltage, would be my guess. Opposite of what occurs during charging of 2 unmatched batteries in series, as described above, for nickel and Lead systems. Old days, rugged, flooded, vented nickel cadmium could be run at overcharge without damage from any dumb charger until all individual cells were brought up to maximum potential.. they had the ability to be run at extremely high discharge current close to Short Circuit levels without damage almost like a super cap. I experimented for a Time with some NiCd Army Surplus helicopter starting batteries, that I purchased cheap. But I don't think they make those anymore because of cadmium toxicity, corrosion from the sodium hydroxide electrolyte, and maintenance like having to add distilled the water..

 

Yes, that can definitely happen in a 24V bank made from two 12V batteries in series. Once the two batteries start drifting apart in state of charge, one can reach full sooner and get pushed harder on charge, while the other lags behind. Over time that imbalance can keep getting worse.

That’s one of the main reasons people either balance them periodically, charge the 12V batteries individually from time to time, or move to a proper 24V LiFePO4 pack with a good BMS. I’d also check that no small loads are being taken off just one battery instead of the full 24V bank, because that can slowly pull the pair out of balance too.