Battery tester

[donkey]

There is a calculation method, which has a few steps to give you the true battery capacity. First charge the battery to 100%. Then take out a known number of amp-hours. Let the battery rest and measure the resting voltage.

Use the chart to find the remaining % charge in the battery. Subtract from 100 to find the % charge taken out by the load, ie the percentage fall from the 100% level..

The Amp-hours drawn (A) represents the percentage fall (P) of the true charge capacity (C).
A amp-hours is P % of C, or as a formula,
A = (P/100) x C which can be rearranged to give
C = 100 x A / P

True Capacity = 100 x Amp-hours drawn / Percentage fall.

For example, if you take 40Ah out of a full battery, and its final charger level is 55%, then the charge level has fallen by 100 - 55 = 45%. The true capacity is 100 x 40 / 45 = 88Ah.

So if it's 90Ah on the battery label, it's fine. If it's 110Ah on the label then it's more of a concern. the capacity has dropped to 100 x 88 / 110 = 80% of its label value.

Hi
I am swayed towards using this method, as it can be completed more quickly and probably more accurate than the one I use. So, could I please ask for clarification of a few points?
Is the chart you refer to, the one posted my Lenny?
I assume this is no use for Lithium batteries, so is there a similar method for those. I am thinking more LiPo & Li-ion, as I have dozens of those that I would like to test for capacity.

Geoff

 

[bigtwin]

I assume this is no use for Lithium batteries, so is there a similar method for those. I am thinking more LiPo & Li-ion, as I have dozens of those that I would like to test for capacity.

Correct. LiFePO4 batteries have a very flat voltage curve so you cannot use voltage to assist in this respect.

For LiFePO4, I believe, to capacity test you need to effectively completely discharge the battery and measure Ah discharged.

Ian

 

[autorouter]

Is the chart you refer to, the one posted my Lenny?

The chart is the greatest part of the uncertainty in this method. Ideally you should use a chart provided by the battery manufacturer, but mostly they are not available. Then you just have to find a chart for your battery chemistry type, preferably one that others have found reliable. So yes, I'd use the one posted by Lenny HB, after looking at a few others on the internet to see if they were in agreement.

 

[The Artilleryman]

Bloody hell ,I never knew battery condition testing was so complex .Is there a degree out there that I can get

Just try matching 6 cell (sub c) 7.2V packs, where you really do want the last mah out of it.
Radio control model car racing, started with 1000 mah SC and left the hobby with 3600 mah NiMH, so probably 12 years ago.