Cell 1 has much lower voltage in two batteries when discharging

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pmbakker asked Jul 28, '22

Cell 1 has much lower voltage in two batteries when discharging

Hi,

In my campervan I have two 200Ah smart lithium batteries connected in parallel. I also have a multiplus 3000 connected to provide 230V for an induction hob and electric boiler.

I’m noticing that when I have the cooktop on Cell 1 has a much lower voltage than the others and will cause an undervoltage shutdown even though the total SOC is between 70-80%. This is true for both batteries.

The voltage difference will be greater if the load increases. I try to keep the load under 1500W total. When I switch off the cooktop, all cells will have about the same voltage again, but the batteries will be imbalanced after one or two days of use.

It is inconvenient that I cannot use the full capacity and have to recharge/rebalance every other day.

Does anyone know why this happens and if there is a solution?

Lithium Battery

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Stefanie (Victron Energy Staff) ♦♦ commented · Jul 28, 2022 at 02:50 PM

@pmbakker

More input required. Which BMS and charge sources are you using? What's the source for SoC?

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pmbakker Stefanie (Victron Energy Staff) ♦♦ commented · Jul 28, 2022 at 03:08 PM

Hi Stefanie,

I’m using the VE.Bus BMS to control the batteries and a SmartShunt 500 to determine the SOC.

As charging sources, I have the Multiplus 3000 and a victron buck boost 50A to charge off the car’s alternator.

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ianddindustries commented · Jul 29, 2022 at 11:18 PM

Hi. I have had this issue before and it was a loose bus bar between the cells internally in that battery. If it’s the newer style of battery with the screw top I would remove the lid and just check the tightness of the connections. Regards

Grant

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Answer 1 · 2022-07-28T12:05:49Z

klim8skeptic answered · Jul 28, 2022 at 12:05 PM

That is a fair bit of voltage difference between the cells.

It would be a good idea to pull that battery out and give it a decent extended charge by itself. Hold the absorb voltage at 14.2v for 24 hours or so, and limit current to 1-2a once in absorb.

Charge the remaining battery fully, before placing the offending battery back in service.

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pmbakker commented · Jul 28, 2022 at 02:20 PM

Thanks for your quick reply. Because this happens in both batteries though I’m not sure if both batteries have an issue with cell 1 or if it is something related to the system or the type of load.

I also see that while charging the system to resolve the imbalance, cell 1 (in both batteries) will reach an overvoltage shutdown situation and cause temporary ‘charge disabled’ states.

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Answer 2 · 2022-07-28T15:59:36Z

Stefanie (Victron Energy Staff) answered · Jul 28, 2022 at 03:59 PM

Thanks!

There is a whole chapter in the manual which deals with cell imbalance and how to recover an imbalanced cell, just in case you didn't read it.

One important thing to know is that if you are discharging the battery with high loads almost every day, it will also require an almost daily full charge cycle, which includes cell balancing at 14.20V absorption voltage for 2 hours (even if it appears that all cells are in balance) to give the cells the necessary time to balance. Such a huge imbalance that you see can take even longer to resolve, up to a day or more as klim8skeptic pointed out.

There is a calculation example in the same chapter, to give you an idea how long this process really can take.

A cell overvoltage event (<3.75V) should recover on its own once the cell voltage falls below 3.70V, and may recur until the faulty cell is well in balance.

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pmbakker commented · Jul 28, 2022 at 04:51 PM

Hi Stefanie,

Thanks for your reply!

I have read the manual, but I guess I didn't think a load of about 100A divided across two parallel connected 200Ah batteries was considered to be high and would require full charging cycles so often. It seems I have to change my expectations about how long I can stay off grid a bit ;-)

I will also increase the absorption time in the Multiplus from the current 1 hour to 2 hours and restart the charger more often when I'm connected to power.

Kind regards,

Peter

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Stefanie (Victron Energy Staff) ♦♦ pmbakker commented · Jul 28, 2022 at 06:16 PM

Hi Peter,

Also I wouldn't consider 100A a really big load for a 400Ah battery. But it is also about the depth of discharge, discharge current, how often the battery gets discharged (every day or a few times per week), charge voltage (14.20V), time in absorption and so on. See also How to ensure that the battery remains balanced in the same manual mentioned above.

When I'm not on board my sailing yacht (has the same battery capacity with 4x100Ah smart lithiums), the BMS triggers a full charge cycle every 30 days with a constant discharge current of around 2.5Ah, whereas when I'm on board, the power consumption increases rapidly (induction hob, oven, hair dryer, watermaker, heating, etc.) with a discharge depth of around 20-35% per day. Thanks to PV, however, the battery gets a full charge almost every other day or so. No problems so far in the last 3 years.

Maybe solar is something you could consider too.

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pmbakker Stefanie (Victron Energy Staff) ♦♦ commented · Jul 28, 2022 at 08:19 PM

That sounds like a simular scenario then. Adding solar is an option, but I was hoping that the buck boost would keep the batteries fully charged during longer drives. It doesn’t support a charging profile though, so maybe I picked the wrong device and should switch to an Orion Smart DC - DC charger which does support this?

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Stefanie (Victron Energy Staff) ♦♦ pmbakker commented · Jul 29, 2022 at 09:12 AM

Define fully charged ;-)

The Buck Boost works in a similar fashion as the Orion Smart with the difference that the Orion has defined charge algorithms. I doubt it will make much of a difference.

Let's say your batteries are at 30% SoC when you start driving. You would have to ride for more than 10 hours, constantly charging the batteries at 25A, plus another 2 hours to properly balance them. How likely is that? And don't forget that intelligent alternators often switch off in between.

In my opinion, the best thing would be a charging source that keeps the batteries at a sufficient level when not driving.

The combination of Solar, DC-DC/Buck Boost and MultiPlus charger makes the difference, not a single charging source.

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pmbakker Stefanie (Victron Energy Staff) ♦♦ commented · Jul 29, 2022 at 08:15 PM

Thanks again for your advice!

I’ll look into adding solar then.

Kind regards,

Peter

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