Maximum voltage capacity

[AKTroy]

So, beginners question here. I purchased eight 304 AH Eve cells, Which are wired up in series for a 24 volt system. The BMS is I believe the same brand that Overkill solar sells.

I initially top balanced the entire pack in parallel up to about 3.58 volts. All good.

When I go to charge the pack it seems to cut off right around 3.4 volts per cell, which seems on the low side. I bumped up the maximum voltage to 3.65, with an over voltage protection I believe of 3.9 in the BMS. Didn't seem to make a difference at all. Still cuts off at about 3.4 volts. Cell voltage differential is .010

I guess this is great in one sense that the BMS is not allowing over charging, but would still like to get the volts up to around 3.5, yet I'm at a loss to explain why the I can't get any higher than 3.4.

I've gone through all the parameters settings in the BMS multiple times, and it doesn't seem like I'm missing anything, but I'm posting here in the novice section so I am certainly no pro at this. Could anyone give me any guidelines as to what to try next? My understanding is I should be able to safely go up to 3.65 volts per cell, although I would get a longer life if I went lower. Somewhere in the 3.5 range is what I'm shooting for.

Thanks,


Troy

 

[sunshine_eggo]

So, beginners question here. I purchased eight 304 AH Eve cells, Which are wired up in series for a 24 volt system. The BMS is I believe the same brand that Overkill solar sells.

I initially top balanced the entire pack in parallel up to about 3.58 volts. All good.

When I go to charge the pack it seems to cut off right around 3.4 volts per cell, which seems on the low side. I bumped up the maximum voltage to 3.65, with an over voltage protection I believe of 3.9 in the BMS. Didn't seem to make a difference at all. Still cuts off at about 3.4 volts. Cell voltage differential is .010

I guess this is great in one sense that the BMS is not allowing over charging, but would still like to get the volts up to around 3.5, yet I'm at a loss to explain why the I can't get any higher than 3.4.

I've gone through all the parameters settings in the BMS multiple times, and it doesn't seem like I'm missing anything, but I'm posting here in the novice section so I am certainly no pro at this. Could anyone give me any guidelines as to what to try next? My understanding is I should be able to safely go up to 3.65 volts per cell, although I would get a longer life if I went lower. Somewhere in the 3.5 range is what I'm shooting for.

Thanks,


Troy

Pull your over-voltage protection back to nothing higher than 3.65V.

There is also a setting for total pack voltage. That should be set to something like 29.2V.

By "cuts off", are you saying the BMS terminates charging?

 

[AKTroy]

Yes to the BMS stopping the charging or cutting it off IOW. Overvoltage is set to 3.65, pack voltage set to 29.2. Charger powers way down at 3.5 and then stablizes at 3.384 and .007 voltage diff. I will take a closer look at the charger settings tomorrow, as maybe that's the issue?. I'm using an all in one MPP solar hybrid unit for the charging. All this is very new to be, so appreciate the feedback! I'm learning slowly.

 

[sunshine_eggo]

If everything is as you say, it sounds like a defective BMS. Always worth double checking connections just in case.

 

[AKTroy]

If everything is as you say, it sounds like a defective BMS. Always worth double checking connections just in case.

I think I traced it back to the settings in the MPP solar all in one Inverter/charger. I'm not sure if I should just put it on auto, and let the BMS manage the total charging or set the maximum voltage in the MPP solar/ Inverter. I notice if I set the MPP solar at 3.6, The cells (or at least 1 cell) do in fact get up to that level, and then it goes into a discharge mode. I'm assuming to not overcharge any individual cell. Weird thing is the system doesn't stabilize at 3.6 or 3.5 it stabilizes more like 3.3. That's a little confusing to me.

 

[sunshine_eggo]

I think I traced it back to the settings in the MPP solar all in one Inverter/charger. I'm not sure if I should just put it on auto, and let the BMS manage the total charging or set the maximum voltage in the MPP solar/ Inverter

NEVER. The BMS exists to protect the cells when something goes wrong. The equipment should be programmed to operate inside the BMS limits.

I notice if I set the MPP solar at 3.6,

That's pushing it too hard. Balance needs to be perfect.

If you think the BMS is good, set MPP to 3.45V, and see how it goes.

The cells (or at least 1 cell) do in fact get up to that level, and then it goes into a discharge mode. I'm assuming to not overcharge any individual cell. Weird thing is the system doesn't stabilize at 3.6 or 3.5 it stabilizes more like 3.3. That's a little confusing to me.

If you're saying it charges to 3.5-3.6 and settles around 3.3, that's may be normal. LFP is similar to lead acid in that it needs to be taken to a higher voltage to fully charge.

 

[AKTroy]

I did have the MPP solar set at 3.5. It didn't make any difference as to the final stabilized voltage. Is still settles around 3.3. The voltage differential is pretty tiny. Something like .002, so I think the cells are pretty well balanced. I did top balance them before starting. Again, during the actual charging a particular individual cell will get up to 3.6 (My setpoint), and then the whole thing goes into a discharge mode.

 

[AKTroy]

I really prefer to get the bank fully charged as quickly as possible. When it goes into it's discharge / standby mode it's using power to run those fans, (60 watts or so) and doesn't seem terribly efficient. Would be nice just to get up to full charge and then shut off. It doesn't do that though. It's hits the set point of 3.6 and then it goes into this discharge, kind of trickle mode. I don't know, maybe that's normal and it's just the fact I have a BMS now to actually watch this. The MPP solar is not exactly very efficient in some regards. Whether I'm using solar, shore power or DC to DC charging the unit consumes about 60 watts of power to run those fans. Considering my refrigerator uses only 23 watts that seems like a substantial amount to me.