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Lead Acid Battery Load Testing

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Hi All 

Hope some one can advise. I have a 5 KVA RCT (Axpert) 48V inverter. Currently I am running 4 x 12v 90 AH batteries but I think one or 2 of them have died. When in charge it goes up to the predetermined 54v but when we shed the voltage drops to about 50v. The draw is about 500w which used to last 6- 8 hours (plus) , now its about 90 min. 

I would like to know if there is a way to run a stress test or similar to see the battery health? 


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A simple test would be (with fully charged batteries) to connect the batteries to a load. Use 4 automotive headlamps in series.

Monitor the voltage across each battery (and current if possible). As the batteries discharge the voltage will drop. All the batteries should be the same voltage (approx). If they differ then the one that's different is probably kaput.

Another test when the inverter is working is to check the temperature of the batteries (those Covid thermometers are ideal!) Hot batteries mean trouble!


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Sadly JD it sounds like your situation is another tale of Lead Acid battery premature death...

I find the most accurate  method to test a battery is with a capacity tester. Mine has a hall effect current sensor but a current shunt model would work just as well. 

To test a battery I first charge it to full capacity and then test discharge at C/10 which is common for manufacturers to use as the discharge rate for the rated capacity.

I program  the tester to automatically stop discharging at 10.5 V.

I read the battery AH capacity on the the tester as per photo below. (interestingly the camera caught the voltage display as it was transitioning and the old value is "ghosting")

We can see that the battery capacity of the battery tested as 27 AH but the rating is supposed to be 105 AH. 

Sadly this batter is under 2 years old and were not used in a ESS (Energy Storage) application.  

A repeat test got similar low capacity. 


More LA batteries for the scrap metal merchant to come and collect - R9 per kg is going rate. 

So JD I recommend you consider using a Lithium battery for your replacement and you will be sorted for a good number of years. 

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  • 4 weeks later...

Hello, I've got the green RCT 5kva MKSII PF1 (I think). However I use it with 4 200ah Deltech lead acid batteries (in series of course). I believe 200ah was the recommended minimum for your/my inverter. Additionally I bought 3 Victron battery balancers. They seem to work well. I have a 0.1v difference between each neighboring battery. This can prevent premature failure of a 48v system like yours/mine. Of course going forward try change to Lithium. I've attached a picture of such a balancer.


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