I'm asking here in case any of you can offer some insight,

I have 4 x 110AH 12v Leisure deep-cycle lead-acid batteries; wires in series and parallel to bring the array voltage to 24v.

I have 200w PV (2 panels in series), which feed into a 20A MPPT EPVER controller.

The controller is configured for "Flooded" battery type with a total capacity of 220aH The battery array wiring is as follows (pardon the terrible text representation of the wiring), each is 12v, lead-acid, deep-cycle/leisure battery

Key: +'ve |- X -| -'ve

Parallel wiring

   A -|
|-B  |
| C -|

Series wiring

+   -
   B -|
|- C
D -|

Additional details MPPT Solar feed is connected to the A (-'ve) and D (+'ve) Batteries These are matched vendor and model batteries, as such all should perform near identically; PV is two panels wired in series. 3KW Inverter is connected to the same terminals as the MPPT solar feed.

Issues observed For a few weeks, this has operated just fine, though it has been cloudy with little sun where I am; as the array is nearing 95-98% charge (as I've not been using the stored power due to the lack of sunlight) I'm starting to see some odd behaviour.

  1. When the MPPT feed is pushing ~4.5A the voltage across cells A and D can be read on a multimeter as 15.9v / 16v this appears to be too high.
  2. Batteries B and C read at 13.4v and 12.8v this too seems odd, as the wiring setup should be self-balancing.
  3. When MPPT is disconnected from the array by the isolator voltages return to be equal across each battery, as the batteries balance each other (13.2v) My current conclusion is there is that I am not connecting to the right terminals for a balanced charge, so my questions are as follows. BAttery wiring diagram Battery wiring, PV and Controller connection wiring

Is my wiring of the battery array the most efficient and correct, and if not what would be your recommendations on the wiring configuration?

Many thanks in advance.


1 Answer 1


If you're not using a BMS and your configuration is unbalanced cell capacity and ESR, you need to test and make changes.

I recommend you test of the capacity And ESR with some kind of tester or load test and then match these parameters for the best pair and use a BMS to extend the life. You always want the batteries to have shared charging.

For 2S2P arrays , 1st connect 2P with matched sum of capacity then connect in series.


There is no battery equalizer in your diagram of wiring. If the results are unequal. The batteries are unbalanced and there is no balancer. Batteries have 3 parameters to balance. C, ESR and leakage. All these can be measured easily by a tech.

  • During discharging or charging measuring dV and dt then computing C= Ic dt/dV (Farads usually xx kilofarads) the change in voltage over 1000 seconds These made balanced << 0.1% in each cell in a 6 or 12 cell battery. Your packs must be < 1% if they are truly balanced. This is hard to do, so an active balancer is mandatory for each pairs, not just a BMS for the whole pack.
  • high pulse current then measure the small voltage change short duration in milliseconds ESR = dV/I Ohms (usually x milliohms)
  • leakage Resistance is same as self discharge rate in days after after fully charged (weeks or months)

The results also indicate the batteries are not being balanced as told and not fully-charged equally. The schematic also shows NO BALANCER. This array wiring does not do it but it helps a little.

One pair may have been over-charged and the other under-charged when the cutoff total voltage was reached. Active cell voltage string balanced means each voltage is monitored during charge and discharge and either shunted to prevent over voltage or the other shunted to prevent undervoltage. When this is done, these over-voltage conditions which cause rapid aging are prevented. The weaker pair accelerates getting weaker. (weakest link theory)

Sorry if I assumed you knew the job of a BMS is more than just measure pack voltages as a whole, it must protect each pair in series too. Small BMS's can have 6 in series and each must be monitored and protected. An e-car will have many.

  • \$\begingroup\$ UNfortunately no BMS in place, though I am researching how to do this on lead-acid batteries (and finding nothing but BMS for LiPo's) I have disocnnected the whole setup via the isolators so I can test and debug over the next few evenings. w.r.t the 2S2P I take it you're referring to A-B and C-D in parallel as two "banks" connecting the banks in series, or am I not following ? \$\endgroup\$
    – Oneiroi
    Commented Aug 25, 2021 at 15:38
  • 1
    \$\begingroup\$ yes thats correct , but test each to match pair sum \$\endgroup\$ Commented Aug 25, 2021 at 21:36
  • \$\begingroup\$ I have implemenmted a battery equalizer. And ensured this balanced all batteries whilst not drawing power and whilst not being charged. I Charged this with the equaliser in place off of a steady mains charger. I approached a specialist company for advice on the wiring configuration (their recommendation matches my own), I intend to replace the controller within a year as this may be a remaining issue as the volatege being pushed to the battery array was not suitable. Currently returned to service this is working well once more. \$\endgroup\$
    – Oneiroi
    Commented Sep 2, 2021 at 8:34
  • \$\begingroup\$ To update on this, I have replaced the battery array with Hankook deep-cycle leisure batteries XV110, I worked with the vendor to get the appropriate voltages for the models used: Equalize 15.9v, Boost 14.4v, Float 13.8v, Low voltage reconnect 11.5v . I had also installed a batter-equializer between the 4 hakook batteries, along with individual volt meters on each. The charge and discharge of all is within 0.1v at all times; and to date I've had no issues once this was moved to hakook batteries vs cheaper "off-brand" lead-acid batteries. \$\endgroup\$
    – Oneiroi
    Commented May 24, 2022 at 12:56
  • 1
    \$\begingroup\$ Both C and ESR matching is important. Pulse charging works better than Equalizing to 15.9 to equalize specific gravity within 0.1% when pulsed with 25 kHz xx ns duration, using flyback, while charging all the time even with load. I have proof. No need to exceed 14.2V \$\endgroup\$ Commented May 24, 2022 at 15:51

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