Is it okay to charge a battery with (much) lower current than recommended?

[tomashubelbauer]

Hey folks, I was recommended to repost my question from https://diysolarforum.com/threads/i...ith-much-lower-current-than-recommended.50780 here:

Long story short, I am a complete novice and I happen to have a 13.6-14.7 V / 1 A charger meant for gel batteries at hand and I also have a new SOK battery, this one:

Marine Grade 12V 206Ah Battery, Sealed Plastic Box Bluetooth&Built-in heater | SOK Battery Europe

EU - Pre order. Will be in stock in mid-May. All pre-order can enjoy - €50 discount. Contact cusotmer service to get the refund. UK - Out of stock. Latest V8 version Bluetooth BMS.SK12V206PH designed for cold weather using! Can safely charge at temperatures down to -20°C (-4°F) by a standard...

www.europe.sokbattery.com

Its spec say it needs 14.4-14.6 V / recommended 40 A to charge. My charger I guess is compatible voltage-wise, but the current is super low. My question is, is this only affect the charge time (even if significantly) or could it damage the battery?

The battery came almost discharged (it seems) but I won't be able to tell for sure until I cycle it twice and the BMS synchronizes and shows the true SOC.

This is mostly me being curious, I don't have a proper charger now but I did order one because I will need it in the future anyway.

 

[time2roll]

14.7 is a bit high but it will charge the battery fine. I would stop around 14.2-14.4
Could take 4 to 8 days to charge. Some chargers will time out and drop to float in 3 or 4 days. Keep an eye on it.

 

[DeeK]

I agree with time2 roll about limiting the V to less than recommended. The issue is that at low charge rates, the V to SOC curve gets very steep as you approach full charge. As you approach 14V at slow charge rate, it takes very small increments of power to cause the V to rise. Your battery has 4 cells that are not perfectly matched and require an internal balancer of some sort (hopefully it has one) to keep their voltages close. That becomes a much harder task at very low charge rates because any cell(s) start to runaway as they approach full while others are still trying to reach that level.
The recommended 40A charger yields about a 0.2C charge rate. A 1A charger is a 0.005C charge rate which is incredibly slow and I believe could result in one or more cells overcharging only being cutoff when they hit their inherent cutout which may damage them. At that slow rate, I would not be comfortable pushing the battery beyon 13.8 V at the max, 13.6V might be better even 13.6V would be over 99% charged anyway. I can dig up some charge curves that illustrate this relation if it would help.

 

[Ampster]

The issue is that at low charge rates, the V to SOC curve gets very steep as you approach full charge. As you approach 14V at slow charge rate, it takes very small increments of power to cause the V to rise. Your battery has 4 cells that are not perfectly matched and require an internal balancer of some sort (hopefully it has one) to keep their voltages close. That becomes a much harder task at very low charge rates because any cell(s) start to runaway as they approach full while others are still trying to reach that level.

I think the curve is very steep at at any charge rate as you approach the top. The advantage of a lower rate is that it gives the balancing shunts more time to hold the voltage on the runner cells to give the lower cells time to catch up. I had a programmable charger on an EV conversion that had a lower charge rate option. That option was specifically designed to give a pack more time to balance at the top.
However I agree with the general theory that you do not want to often charge at low currents because maintaining cells at high voltage for a long time can be more harmful. That is why the above option was only intended to be used less frequently.

 

[DeeK]

I think the curve is very steep at at any charge rate as you approach the top. The advantage of a lower rate is that it gives the balancing shunts more time to hold the voltage on the runner cells to give the lower cells time to catch up. I had a programmable charger on an EV conversion that had a lower charge rate option. That option was specifically designed to give a pack more time to balance at the top.
However I agree with the general theory that you do not want to often charge at low currents because maintaining cells at high voltage for a long time can be more harmful. That is why the above option was only intended to be used less frequently.

No, the slopes of the charge curves vary dramatically toward the bottom and top depending on charge rate. Charging at 1C has much more gradual slope towards the top which allows you to push 14.6 (in a 4 cell battery) without issue. At 1C though, 13.6v would only be about 70% charged. I'll try to dig up some charge curves tomorrow to illustrate if I can get out of tday duties. I do agree that it's bad practice to keep batteries topped up.
Incidentally, the effect of charge rate is so critical that imo, it's why it's virtually impossible to develop a set of "correct" values for absorb voltage and duration. What's correct at high charge rates is not optimal at low rates and vice versa.

 

[DeeK]

I wish I could provide credit for this work but I cannot remember who generated these curves. I believe it was someone on this forum though...Apologies to that person because these are extremely useful.

These only cover the range typical in domestic solar charging from 0.2C to 0.05C but the steepening at the top end is obvious.

At the OP's recommended charge rate of about 0.2C, his cells would be less than 70% full at 3.4V/cell (13.6V battery) whereas at a charge rate of 0.05C, the cells would be >95% full at 3.4V/cell (13.6V battery). The curves get much flatter at very high charge rates and the top section goes essentially vertical above 3.4V/cell at very low rates. Therein lies the problem with maintaining cell balance at very low charge rates.

 

[John Frum]

Long story short, I am a complete novice and I happen to have a 13.6-14.7 V / 1 A charger meant for gel batteries at hand and I also have a new SOK battery, this one:

Do you mean the charge voltage is configurable between 13.6 and 14.7 volts?
Or do you mean that it has a bulk/absorb voltage of 14.7 volts and a float of 13.6 volts?
Or something else?

 

[tomashubelbauer]

Do you mean the charge voltage is configurable between 13.6 and 14.7 volts?
Or do you mean that it has a bulk/absorb voltage of 14.7 volts and a float of 13.6 volts?
Or something else?

I found this weird, but the label on the charger says its output is 13.6-14.7 V / 1 A. I am confused why it would be a range. It is not an LFP charger so it does not state any specific voltages at the different charging stages like bulk and absorbtion.

 

[John Frum]

I found this weird, but the label on the charger says its output is 13.6-14.7 V / 1 A. I am confused why it would be a range. It is not an LFP charger so it does not state any specific voltages at the different charging stages like bulk and absorbtion.

bulk/absorb are not unique to LFP.

 

[tomashubelbauer]

bulk/absorb are not unique to LFP.

I see. Still, I would have expected to see those values represented not as a range but as lines showing the voltages for the different states. I am attaching a picture of the label.

 

[John Frum]

I see. Still, I would have expected to see those values represented not as a range but as lines showing the voltages for the different states. I am attaching a picture of the label.

That charger is rated at 1 amp.
It will take 206 hours to fully charge that battery.
I would check the voltage with your reference meter.

 

[tomashubelbauer]

That charger is rated at 1 amp.
It will take 206 hours to fully charge that battery.
I would check the voltage with your reference meter.

Yeah I left out some of the context from my original thread. My situation is that I'm not at my house where the solar system is now, but I did receive this battery and rather than just stare at it for a few weeks, I figured I might as well do the first two cycles as recommended by the manufacturer. I don't have any gear where I am not but this charger and time which prompted this question. After seeing the replies though I have decided to get a Victron wall charger instead, because while I would have been happy to wait 206 hours for the battery to charge with this one, I wouldn't be too happy to see the cells disbalanced. I plan on buying more batteries with time so the wall charger will come in handy in the future so I decided to go for it. I was still curious about the risks of super low C charging though so I put this question up to learn.

 

[John Frum]

.

 

[Ampster]

The curves get much flatter at very high charge rates and the top section goes essentially vertical above 3.4V/cell at very low rates. Therein lies the problem with maintaining cell balance at very low charge rates.

If one were to plot voltage with time on the horizontal axis it would a different shaped curve. To my point if giving the BMS more time for balancing, using a lower current once the balancing starts at 3.4 volts would allow more time for the low SOC cells to catch up. This assumes the current shunted on the high SOC cells is about equal to the charging current applied.

I also assume that chart is only reflective of a constant current (Bulk) and does not account for a Constant Voltage (Absorb stage). I am not advocating this for everyday charging. The ideal scenario would be to charge at .5C until the balancing shunts start working, then drop the current until the cells with lower SOC can catch up. Not many chargers can be programmed this way but it is a methodology to get a pack balanced in fewer cycles if one can implement it. However the typical CC/CV cycle by its nature, starts out with a higher current than it finishes with and that is what most BMSs use for the balancing stage.
Another point to consider is that Winston LFPs are a different formulation with Ytrium and since most users here are using much more dense cells by EVE, CATL and others. I had an EV conversion with those Winston cells and they settled at 3.2 volts versus the 3.3 volts I observe with my EVE cells.

 

[JoeHam]

I would have been happy to wait 206 hours for the battery to charge with this one, I wouldn't be too happy to see the cells disbalanced.

I was still curious about the risks of super low C charging though so I put this question up to learn.

You would actually be less likely to see the cells unbalanced at the 1 Amp charging rate. Your SOK BMS is likely better able to keep all cells inline at a slow charge rate since there is a limit to how much current it can bleed off or redistribute whether it is an active or passive BMS.

There is no problem with a slow charge C rate.

The main concern was limiting the upper voltage to less than 14.7 Volts.

 

[tomashubelbauer]

You would actually be less likely to see the cells unbalanced at the 1 Amp charging rate.

You're right, I misinterpreted some of the earlier replies.

The main concern was limiting the upper voltage to less than 14.7 Volts.

Yeah, I think this is ultimately why I'll wait for the Victron wall charger to come before I do anything with the battery. The charger that I have on hand seems very odd to me. I still don't get why there is a range of voltage at the output according to its label. I could measure the actual voltage it gives, but I am not sure I can trust it to stay within the safe range for the battery even if it might seem so based on the measurement.

 

[JoeHam]

Your battery, your choice.

However, I would use it to get started and monitor the voltage as it goes up. You could probably run it for a couple of days before you even get to 13.8V.

You could stop there and be fine. Top it off with your Victron when it arrives if you wish but there is very little energy stored above 3.45V per cell.

And, welcome to the forum. Keep asking questions until you’ve learned enough to answer a few ?.

 

[tomashubelbauer]

I didn't consider that, it is true that I can stop while still well within the safe levels and switch chargers once able to. I think I'll do that. Thanks!

 

[DeeK]

If one were to plot voltage with time on the horizontal axis it would a different shaped curve. To my point if giving the BMS more time for balancing, using a lower current once the balancing starts at 3.4 volts would allow more time for the low SOC cells to catch up. This assumes the current shunted on the high SOC cells is about equal to the charging current applied.

I also assume that chart is only reflective of a constant current (Bulk) and does not account for a Constant Voltage (Absorb stage). I am not advocating this for everyday charging. The ideal scenario would be to charge at .5C until the balancing shunts start working, then drop the current until the cells with lower SOC can catch up. Not many chargers can be programmed this way but it is a methodology to get a pack balanced in fewer cycles if one can implement it. However the typical CC/CV cycle by its nature, starts out with a higher current than it finishes with and that is what most BMSs use for the balancing stage.
Another point to consider is that Winston LFPs are a different formulation with Ytrium and since most users here are using much more dense cells by EVE, CATL and others. I had an EV conversion with those Winston cells and they settled at 3.2 volts versus the 3.3 volts I observe with my EVE cells.

I don't know enough to comment on how the different formulations would affect charge curves but I have watched intently through an embarrassing number of charge events for different LFP batteries and I have found these curves to be very accurate and representative. I have unfortunately not found that longer absorbs provide better balancing. In fact, where cells are not well matched to each other, I find the opposite to be true. A cell that is nearing full charge will start to runaway when it exceeds 3.48 V (or so) and nearby cells actually start losing V to it! It's as though the runaway cell is parasitic on the less full cells and it only gets worse the longer they absorb. I have tried several balancers of different designs and balancing currents and this has remained a problem. The only solution I have found is to match the cells very closely, top balance them, and then "condition" them by very gradually increasing the absorb V and time. This has gradually, over a period of weeks allowed me to push them to higher absorb V and times. I also finally found a balancer that is much more effective than earlier units I tried and, it is also the cheapest!