Good source of used/blemished batteries

tallgirl

Re: Good source of used/blemished batteries

AntronX wrote: »

Since I tested it at 30A, my result is lower than if I were to test it at 25A. That's why real percentage compared to factory test should be slightly higher than 91.4%. I wanted to do real 8 hour rate test (255 Ah / 8 h = 31.8 A ) Turns out the real real 8 hour test rate is 25A, because I was basing my Ah value on advertised "20 hr" rate. Interestingly, even at 8A spec sheet rate, they still fail to reach advertised 255Ah. 1670 minutes / 60 = 27.8 hours ! Way more than 20 hrs.

You have to have (or calculate) the Peukert exponent to know the capacity of the batteries at different rates of discharge. You can't assume the C/20 rate applies to anything other than C/20 for the discharge current.

You also can't use an AC load since the DC current increases as the DC voltage decreases.

rplarry

Re: Good source of used/blemished batteries

Why would you want to pop their bubble?

AntronX

Re: Good source of used/blemished batteries

tallgirl wrote: »

You have to have (or calculate) the Peukert exponent to know the capacity of the batteries at different rates of discharge. You can't assume the C/20 rate applies to anything other than C/20 for the discharge current.

You also can't use an AC load since the DC current increases as the DC voltage decreases.

I don't need to calculate or know Peukert exponent when I have factory data for different rates of discharge. I just pick one Amp rate and stick to it, then compare the time it took to get to 10.5V with the time in factory data sheet. I should have used 25A load instead of 30A, but that only made my results slightly pessimistic.

About AC load, sure I can use it. You are right that at constant power as voltage goes down, current increases. If at 12.5V I=30A, then at 10.5V I=35.7A. But I varied my AC load during discharging process. I turned on and off various AC loads to keep battery current at 30A. Its been 6 months and my system is still getting efficiency of 92-94% between output of charge controller and DC loads input. According to in and out watt/hr meters data over 1 month. Good enough for me.

BB.

Re: Good source of used/blemished batteries

The constant current draw for C/XX ratings are not practical for most people any more either... About the only "useful" load that is anywhere near constant current in a varying voltage environment is a filament light bulb (the resistance of the bulb goes down and voltage falls--making, more or less, a constant current source).

To "rate" your system by adding placing a constant load (in watts) on an inverter better matches the true loads on their systems today (i.e., most are using AC inverters or other constant power devices--filament lamps are long gone as a majority load).

If you are looking to characterize your system and/or monitor battery/system issues over time--using an inverter with constant power 120/240 VAC load is an easy and accurate (for your needs) method of getting the numbers.

By the way, when looking around at the various battery mfg. sites for questions raised here--I am seeing that (at least some in the) industry has adopted the constant power model to for their UPS batteries. They offer rating charts based on constant power loads.

-Bill