Voltage Setting on Benchtop Power Supply

[Calothrix]

I have a 10A max power supply that is also 30V max. I just discharged my 380Ah, 12V, 2P4S configured LiFePO4 battery and I am in the process of charging it with the power supply. I do not want to charge with any more than 6A, as I do not want to take a chance of burning out the power supply. However, in order to get it to charge at 6A, I have to set the voltage to over 15V. As the battery charges, the current coming from the power supply slowly falls. Is it OK to increase the voltage to keep the current around 6A? Is there a max voltage setting that I should not exceed in order to accomplish this? Basically, what I am asking, is it OK to crank up the voltage early in the charging process to speed things up a little bit and then once the battery is charged to about ~13.3V or so, turn the voltage of the power supply down and monitor closely until it is fully charged? Thanks!

 

[gnubie]

Usually a charger will be set to pull up to a higher voltage, say 14.4 for a 12V lifepo4. You'd set your current limit at what ever you like and then the battery voltage rises up to 14.4 over time. The current flowing into the battery will stay at a fair percentage of your set limit until it approaches 14.4V and then it will fall off pretty quickly due to the lithium battery's charge vs voltage curve.

Once the battery reaches low current at 14.4V, say 3% of the AH rating, you can usually call it a day and turn the charger off. You do want the voltage to be above your BMS' start cell balancing voltage for a while to let it do its thing though.

You can increase the voltage to move things along faster but you'll likely get caught at the end of the charge where it rises up quickly and your BMS steps in with high voltage disconnect. You may find it does this after 14.6V is exceeded.

 

[Calothrix]

Thanks gnubie! I have a battery monitor but not a "BMS" per se. So bottom line, it sounds like it is OK to use a charging voltage that exceeds 14.6V, as long as I turn it down as it approaches the end of the charge cycle, so the total charge of my battery pack does not exceed 14.6V. Is this correct? If so, could I set the bench supply as high as say 20V and the battery still be OK with that? Thanks again!

 

[gnubie]

I'd strongly recommend a BMS. Without it pushing the voltage up places one or more of your cells at real risk of being overcharged and ruined.

Now that I know you don't have a BMS you should not push the voltage up. I would keep it at 13.3.

 

[Zil]

I recommend you do not charge at such high voltages. Especially without a BMS. Have you top balanced the cells? Connect them in parallel and charge with steps up to 3.65 then discharge down to 3.2.

 

[Craig]

You can charge the battery at a fair amount over it's max voltage.whatever voltage you choose if that value only lets 6 amps into your 380ah cell you will be fine. At 6A if cell was empty you would need over 60 hours to charge. The danger comes if the cell itself is allowed over 14.4 volts which will happen if you do not stop charging when it gets there. I use a microcontroller to cut off charging when I am not around. In your case checking on the battery every few hours should suffice. But the closer it gets to full the more careful you need to be.

 

[Calothrix]

Thanks for all the responses! I top balanced the cells when they were arranged in parallel. I then put them in a 2P4S configuration, discharged them (lowest pair of cells was at 2.5V) and now I am fully charging them again. Since these cells are new and I have some time to "play" with them before my campervan is complete, I'm trying to determine their capacity and generally seeing what there response to charging and discharging is as I record voltages, etc. I monitor the pack in three ways: AiLi battery monitor for total pack statistics, BG-8S battery monitor/balancer for paired cell voltages and low voltage alarm, and a DVM for individual cell voltages. As long as I check them often, hopefully I should be OK. Like the low voltage alarm, is there also some kind of inexpensive high voltage alarm that sounds off when the individual cells/battery pack exceeds a set voltage? Craig, you mention a microcontroller to cut off charging. Do you have a link to this item. Thanks again!

 

[gnubie]

The risk is at the end of the charge process because lithium quickly rises in voltage at the end. Check any charge / discharge curve for them and you'll see what I'm talking about. Without something to intervene, ie a BMS, it's an almost guarantee you'll get caught by this if you set your power supply to > 14.6V and don't literally sit there watching the voltage rise. It can happen over a matter of 5 minutes. It won't matter if your cells are all balanced, that just means they'll all spike up in voltage at the same time.

 

[Zil]

Please use a BMS. One of our members sells a decent one. overkillsolar dot com