Deep cycle battery health check

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Joined: Fri Oct 01, 2021 4:06 am

Deep cycle battery health check

Post by Tomblueprintauto »

I've been wondering if there is an accurate way of checking the RC of a deep cycle battery using a scope? Some way of avoiding the 20A discharge tests the take hours.

Perhaps if we hook up a load to the battery and monitor current and voltage over time we could come up with a math channel that will display the RC?

Anyone playing around with this?

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Re: Deep cycle battery health check

Post by ben.martins »

Hi all and apologies for the late reply.

I think we can all say that testing batteries is not an easy task and to quote from Steve Smith’s post - “There is no fool proof method for testing 100% of all battery faults (this includes the impedance test) but we can be on our guard when testing batteries for all vehicles.” viewtopic.php?p=57461#p57461.

When it comes to reserve capacity, RC, we are looking at the batteries capacity to sustain a voltage over a period of time at a fixed current. The standard for this is SAE J537 which requires the battery to be fully charged, rested for a day and then discharged with 25A till the battery voltage has dropped to 10.5V. This can of course be measured with Pico using current and voltage probes and captured over time.

Currently the battery tester in PicoDiagnostics relies on the fact the starter motor will drop the battery voltage whilst pulling current to get the engine to start turning. This helps us to obtain an instantaneous result to obtain an estimated CCA rating but doesn't determine capacity. In fact to ensure the battery is capable, the test should be repeated 2 or 3 times whilst recording the results as most batteries can crank an engine with very little capacity.

If you’ve not heard of it before I would suggest checking out Battery University - You could lose yourself for days with the amount of information on this site. There is one article - ... e-capacity which has an interesting statement -
“Most rapid-testers look at the internal resistance and do a CCA approximation. Reading battery resistance is relatively simple, but this alone cannot predict capacity, nor can it tell when to replace a battery as the end-of-life characteristic is primarily capacity related.”

When it comes to deep cycle batteries there are even more hurdles to get through as a new battery can actually report a lower capacity than indicated. Again a pierce on the Battery University site can give further insight. ... -batteries.

Load testing in my eyes is still the best option to determine a battery's health. I can remember the drop testers where you connect across the battery with a large sprung piece of steel which gradually heated up as the current flowed. This would quickly determine if the battery was capable of sustaining a load but appreciate that despite it keeping your hands warm, the glowing bar in the tester could be regarded as a health and safety risk. What is interesting is we have seen that some electric vehicles are loading the batteries before the move to ready on. Steve found this on the Renault Zoe which uses a pulse box to apply a load on the battery for a set period of time. You can see the full case study here - ... ure-danger.

Using the scope to watch how the battery reacts to a load and then how it recovers can provide you with an indication of battery health. A low voltage drop and slow recovery indicates a failing battery. This of course is dependent on a number of factors such as temperature, battery ageing etc which is why battery testing can not be done with just one test. Formulas and maths can be applied post capturing to help analyse further but hopefully we can all agree that battery testing isn’t straightforward.

I hope this helps.

Kind regards


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