


Help!
Powerlab 8 and large lifepo4 cells discharge test
I know this isn't RC related, but it is battery related and I haven't found another more suitable forum to ask this question on so I'm posting here. I have 16 sinopoly 100ah lifepo4 cells in a 4s4p configuration for a 12v 400ah battery bank for my camper van. I purchased the van with the cells already installed. However there have been a few issues with the BMS detecting a cell group that hits the high voltage cut off so I want to run through a charge and discharge to check the capacity of each cell. It was suggested that I get a powerlab 8 and the PC adapter to graph the results of the test. However I'm unclear on how I should go about discharging. The cells are are recommended to charge and discharge at 33 amps (1/3C) and that's about the max they would see a charge and discharge is probably a little higher with heavy loads (induction cooktop and microwave) but most of the time they are discharged at very low rates. I see that the internal discharge is 100w, but are there current restrictions? since these are 3.2v cells if I discharge them at 30 amps that's 96w. Could that work or is there a limit on the discharge current? This would save me from having to purchase another 12v battery for regen charging.
Also can you graph your results with the powerlab software? I downloaded it but don't have the powerlab unit yet so I'm not sure if I need additional software. Thanks! 






I have the Dual Power Lab 8 which has a few differences but none that I think affect your question. In hooking up the charger to one of my 100 Ah Bricks and running down the list of LiFeP04 presets, I don't see any of the NONBALANCED (no balance harness for 1S) presets that allow over a 20 Amp rate discharge. If you can live with a slower discharge, which will probably give a better ultimate capacity of each cell, then you're good to go at this point. You're charging is also limited to a 20 Amp rate.
Update: OK, so I can get only a 10 Amp discharge (33 Watts) and the charger says it's not limiting anything and cell voltage is way above minimums. It's max'd at 10 Amps out. The Power Labs are certainly one of the best and safest chargers out there but they are very conservative and restrictive in what they allow you to do. This is a case inpoint. Maybe someone more knowledgeable than I, can chime in on the 10 vs 20 Amp discharge rate. The graphing worked fine on discharge. Vertical Voltages on the left, Time along the bottom and the typical data line. I also have an iCharger 4010. There are no restrictions on the icharger for what you want to do. I pulled 33 Amps on internal discharage. However, the internal charger temp continued to rise slowly even with the fan on at it highest speed. I lowered it to 30 Amps and temperature stabilized. They are roughly $350 and you'll need a dc power supply to go with it. (same with the Power Lab) PL8 is about $250. I am not familiar with graphing on the icharger. A quick search shows there maybe 3rd party graphing software available? 

Last edited by BVH; Jan 02, 2018 at 06:18 PM.




Thanks for taking the time to check for me. Can't you make your own presets with the custom settings to allow for a larger discharge? 20 amps would yield better results on paper/graph, but I'm looking for real world capacity checks with a heavier load. And the internal discharge at 10amps makes sense as after I posted I saw on the spec sheet that it was limited to 10amps.
Since I only really want to do a discharge and charge with a single cell and monitor/capture the results, is there a better product that is more suited for what I'm trying to accomplish? 





You can make your own presets but you are limited with what Max's that FMA has allowed users to pick. For all chemistry batteries, you are limited in max charge and discharge when you don't have balance leads inplay.  the balance harness connected to the battery and charger and the charger seeing individual cells. They institute hard ceiling limits. That is really the key with FMA. Use balance leads or live with their imposed limits. This is precisely why I also have the iCharger. Freedom to do what you want within the capabilities of the product. Most if not all RC hobby chargers, were not really designed for very large capacity cells and packs but in the case of FMA, they do work well if you have a balancewired pack.
That's interesting on the discharge since I can "pick" the 20 Amp rate when making the preset and usually with FMA, if you can "pick it", it will do it. Do you have to have graphing? Aren't you just after ending capacity at a given Low Voltage Cutoff? The iCharger display will give you that at the end of the discharge. 

Last edited by BVH; Jan 02, 2018 at 07:09 PM.










I may not be the best one to ask...I have the buy it once mentality. I used to buy midlevel equipment but would always end up going back and buying the upper tier product and spending more in the long run. So my recommendation is for the 4010. It has the higher internal discharge limit of 130 Watts  so more cooling headroom for your 96 Watt load. Feathermerchant, a vendor here sells modified server power supplies at relatively cheap prices. It's a good way to go. I'd buy at least a 24 volt output unit. Maybe 50 Amps to 75 Amps. This spec will allow you to do some serious charging in the future if the need arises. from a typical household outlet.






Quote:
Thanks for all your suggestions. I've found that the icharger 308 can discharge to a resistor bank which might be a good idea as building a bank will be cheaper than buying a second battery for regen discharging since I'm only going to do this once. That will allow me to discharge up to 40 amps. 






Quote:
However I did read that charge current is limited to 2 amp on the PL6 without a balance lead. However that was from 2013 (a post on here), do you know if that's still the case? Maybe a firmware update fixed that? If not, it's not a problem to make a balance wire for the charger. I found a good deal on a PL6 on the forum classifieds, I can get a cheap 12v battery for regen discharge up to 40 amps if that preset is correct. 






There's no provision for balancing a 1S cell therefore, there's no way to utilize a balance harness for 1S and therefore no way to get beyond the 10 Amp discharge limit for nonbalanced packs. That's the FMA conservative/restrictive design. I tried the nonbalanced presets and even though it allows me to choose 20 Amps, it limits me to 10 Amps.






Quote:







I somehow forgot that my DPL8 is, infact running off a PS, not off a LiFeP04 pack like my iCharger. Even though the FMA discharge limit is 100 Watts internal, I wonder why the 3.3 Volt/20 Amp = 66 Watts discharge "request" is being limited to 10 Amps just because I'm running from a PS? Did you come across anything in the manual that documents the 10 Amp limit?






Watts. Volts and Amps are not the same things. Amp limits dictate how much current can be carried by the circuit irrespective of Voltage. Voltage limits dictate how much Voltage can be carried by the circuit irrespective of current. Both are "do not exceed" numbers. Watt limits dictate how much power can be dissipated by the circuit as a function of Voltage * Current. All three are a "whichever comes first" limit, they are not independent variables.
If the the PL8 has a maximum internal discharge power limit of 100W and a current limit of 10A, even though 3.3*20 is 66W, 20A is more than 10A so 10A is where the limit is enforced. The PL8 will do a higher current discharge with regen enabled, because in this situation it's not the circuits inside the charger that have to dissipate the discharge related power, the external battery getting the regen charge takes care of that instead. 


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