UK 12/24V Battery wiring and charging advice

[alexhibbert]

Hello all,

I'd like some advice on wiring up a 24V leisure power system. It seems to work so far, but my wiring doesn't seem to match many other diagrams online. I'm worried that I'm damaging batteries or wasting power.

I have 4x 12V Lead Carbon batteries (all identical). I want to have 24V output, and the ability to turn off half the batteries (for whatever reason) and still use the other two.

I have wired up:

2 x 12V batteries in a series bank. The +ve lead heads through an isolator switch to the +ve distribution busbar. The -ve lead heads to the -ve busbar.
and....
another the same.

So, I have two completely independent isolatable 24V banks, joined at the two busbars.

1. Is this right? So many diagrams seem to have leads linking the banks - why?
2. As they are Lead Carbon, will the batteries in series (so, within each pair), balance each other 'naturally' and safely?
3. As they are joined at the busbars, will one 24V bank try and balance with the other if there's a voltage difference? If so is this inefficient or dangerous?

Discharging sounds fairly straight forward.

For charging, this happens when the batteries are offline, and each 12V battery has its own fast charger (mostly due to the cost and size of chargers that will charge 24V @ 50A vs. two that do 12V @ 50A). Is there anything I need to be careful about? I presume I must ensure all are the same voltage before going back online, or will they safely balance if I don't?

4. Is there somewhere sensible to place an active battery balancer? Perhaps to link each bank of 24V together?

Thanks.

 

[David Anthony]

1. The reason they show leads linking the banks is they are assuming a single 24v supply and you seem to want two separate supplies.
2. Batteries in series don't "balance themselves out". No two batteries are equal so one may discharge more than the other leaving it at say 10v and the other at 11v total of 21 v. When charging the 11v battery will go to 13 V and the 10v to 11v total 24v. This continues to widen the gap until one is constantly overcharged and other under charged, say 8v and 16. This will damage both batteries.
3. No it will balance in parellel and is acceptable as long as each battery is charged.
4. Don't worry about a battery balancer, charging each 12v battery separately is best.
5. Make sure you have a suitably sized fuse for each 24v bank

 

[plugsandsparks]

Nothing fundamentally wrong with series and or parallel, issues arise over time. Is there a way you can get just two big batteries for 24V and hence avoid a parallel connection ? If not, as you are charging them at the 12V level, they should charge up again to the same level. You can then check if each bank is taking the same current from fully charged with a clamp meter. As mentioned as the batteries age , their characteristics will change and as such one bank can start to feed the other when load is disconnected

 

[oracle]

I lived on a boat for 8 years and fitted 4 X 100A batteries in series / parellel and they were still good after 7 years of use when I sold my boat

 

[alexhibbert]

1. The reason they show leads linking the banks is they are assuming a single 24v supply and you seem to want two separate supplies.
2. Batteries in series don't "balance themselves out". No two batteries are equal so one may discharge more than the other leaving it at say 10v and the other at 11v total of 21 v. When charging the 11v battery will go to 13 V and the 10v to 11v total 24v. This continues to widen the gap until one is constantly overcharged and other under charged, say 8v and 16. This will damage both batteries.
3. No it will balance in parellel and is acceptable as long as each battery is charged.
4. Don't worry about a battery balancer, charging each 12v battery separately is best.
5. Make sure you have a suitably sized fuse for each 24v bank

Thanks.

2. So within each bank, if I let a gap of a volt or two appear, the only way to solve this is manually charging one to catch up with the other? Or get a battery balancer?
3. Do you know how fast this happens? Is it efficient? Or will they get hot? It sounds somewhat uncontrolled.

(my confusion is that I also work with lithium batteries, where balancing parallel banks seems to cause fires/a lot of heat/broken batteries)

 

[alexhibbert]

Nothing fundamentally wrong with series and or parallel, issues arise over time. Is there a way you can get just two big batteries for 24V and hence avoid a parallel connection ? If not, as you are charging them at the 12V level, they should charge up again to the same level. You can then check if each bank is taking the same current from fully charged with a clamp meter. As mentioned as the batteries age , their characteristics will change and as such one bank can start to feed the other when load is disconnected

With this 'feeding', is it efficient without adding an active device to control it?

 

[alexhibbert]

I lived on a boat for 8 years and fitted 4 X 100A batteries in series / parellel and they were still good after 7 years of use when I sold my boat

What did you do if through general vagaries of life the batteries grew gaps in charge level between them?

 

[David Anthony]

I measured the voltage across each battery each month and checked the SG with a hydrometer. Any variance and I would half discharge the lot and charge them all in parellel. That kept them all healthy . Living on a boat is very laid back I a little micromanagent was a pleasant diversion

 

[plugsandsparks]

With this 'feeding', is it efficient without adding an active device to control it?

Depends how big a difference, its just like jump starting a car which has a flat battery, when the two batteries are connected together current will flow until the voltages equalise, basically a bad bank will make the good bank look bad as its spending some energy trying to charge a poorly battery. As mentioned, and David summed it up with periodic maintenance to ensure the batteries are still good, you wont notice any problems. Best to connect them up, bang some load on and see what happens

 

[Lucien Nunes]

About separate-charge, series-discharge:

The cells in any lead battery that is charged and discharged cyclically can get out of step over time, i.e. be at different states of charge, due to subtle differences in their chemistry. For example, cells at the ends of a battery tend to run cooler than ones in the middle, which alters their cyclic charge efficiency. Therefore, all normal modern cyclic charge schemes incorporate an equalisation stage (between bulk charge and float) where the voltage is allowed to rise above the theoretical maximum needed for full charge, for limited period of time. During the equalisation stage, current is forced through fully-charged cells to allow any that are lagging to catch up. The duration and voltage limit should ideally be matched to the battery type, in order not to damage the cells that are ahead, by overcharging them abusively.

Equalisation is therefore a compromise: Too long or too high a voltage and the cells that are more fully charged will be cooked, too short or too low a voltage and the others will never be fully charged and therefore underperform and/or lose capacity to sulphation.

A pair of identical batteries always in series, never charged or discharged independently, is effectively one battery. If the two series parts are separated for cyclic charging their characteristics can diverge. However, if the charger settings and temperature conditions are identical, and the charging is always allowed to run to completion through the equalisation stage, each cell will be subjected to the same regime as they would be if all were charged in series. I.e. the equalisation stage tends to override the minor differences in operating conditions between the two series sections.

Make sure the chargers don't subject the two sections to different regimes. If they are temperature compensated, ensure they are at the same temperature or preferably use a thermistor mounted on each specific battery. Make the leads the same resistance and all should remain reasonably symmetrical.

Re. parallel connection, current tends to flow when battery banks in different states of charge are connected in parallel. With lead-acids, the significantly different voltages off and on-charge means that only a small fraction of the capacity transfers at first, because even a fully-charged bank does not have a high enough voltage to significantly charge a depleted one. But any switching or control systems should be designed to handle a reasonable circulating current without either welding relay contacts or blowing fuses.

The transfer of charge is subject to the same cyclic energy loss as the normal charge/discharge process. So if you have a 90% cyclic efficiency, any charge that transfers will only yield 90% of 90% i.e. 81% to the load. In practice it's a minor consideration unless the banks are often going to be coupled together at very different SoC. If so, it might be worth considering coupling the banks to the load busbar via 'ideal diodes,' which are MOSFET-based switches that behave like diodes with much less voltage drop. They would allow the load to draw current from the higher battery until equal, then from the two equally, but block circulating current. To be clear, it's not usually necessary to do so.