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Topic: LifePO4 voltage Sag under load (Read 3089 times) previous topic - next topic

LifePO4 voltage Sag under load

I did a little torture test with the new batteries yesterday... The coach is in storage (no hookups) and around 400 watts of solar. The 12V marine fridge is on all the time and not much else. The perception that Lithium based batteries have less voltage drop over the course of a charge cycle is true, but they do experience sag under load like AGM's, just not as much. And when the load is removed, the voltage snaps back to very near wear it was before the load was applied. The new 300AH Ampere Time batteries have 200A BMS, are rated for a 200 amp continuous charge/discharge and discharge of 400 amps for 5 seconds. Since I have two of these in parallel, these should be able to discharge at 400 amps continuous, which is a good match for my 3KW inverter (Magnum MSH3012M) which is rated for 400A input.

The test I ran was to go down in the morning after the the batteries were at 100% SOC the previous afternoon. After the fridge and whatever parasitic loads on all night, the battery monitor showed an SOC of 91%
I turned the inverter on and then a kettle (~11 amps) and a 1500W space heater. There was a total current draw that varied between 285 and 295 amps for about 5 minutes. Afterwards, the history in the ARC50 remote showed a low 12.01V. As soon as removed the loads the voltage bounced instantly back to 13.2V, the state of charge on the magnum BM was still showing 90%

I was surprised that the voltage sagged as much as it did, though a similar test with the 4 8d AGM's would have shown a much greater drop and much slower recovery. The Ampere Time manual suggests a low voltage disconnect setting of 10.8V which shows a built in allowance for sag under heavy loads. 10.8V is considered a 1% SOC, so as long as the sag doesn't drop to that level under load, it should be safe to assume that there is a greater than 1% SOC left in the tank.  Accurate assessment for SOC according to voltage would require removal from charging or loading for at least 15 minutes (according to the manual). I am feeling like I got what paid for so far with these batteries... I think two more of the same will be in our future (total of 1200AH!), as well as more solar. and Oh, a bigger alternator... or maybe two. Make it stop! :o
Don
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Don & Tys
1999 U270 3602 WTFE #5402
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Freedom is NOT "just another word for nothing left to lose"... with apologies to Kris Kristofferson