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13/4/2012 at 1:19pm
Location: West Yorkshire
Outfit: Elddis Avante
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Drop test only tests resistance in high current circuits (especially resistance through the battery which is relevant to engine starting scenarios). This has no relevance to it's capacity under long / low discharge conditions.
1. Fully charge the battery and ensure you get to 12.7V on a digital meter after the surface charge has dissipated.
(If it reads higher immediately after charging, all well and good but this is a false reading. You can leave it for an hour or two, or apply a heavy load briefly to bring this down. If you don't get to 12.7V the battery plates are already calcified reducing it's capacity - usually due to leaving a battery flat or semi-discharged.)
2. Apply a known load and see how many hours it takes to get down to 12.2V. Multiply the hours by current drawn by that load and you will have the capacity in Ampere/Hours of 50% discharge.
(Full discharge is 11.7V, but no need to go there, in fact it'd recommended not to. If you routinely exceed 50% your battery is going to expire prematurely, as it will if you don't recharge promptly and keep it charged as much as possible.)
Example:
2 x 25W interior lamps = 50W
50W divided by 12V (nominal) = 4.17 Amperes
Time taken to discharge from 12.7 to 12.2 = 10hrs
10 X 4.17 = 41.7 Ampere/Hours (this is 50% of capacity)
41.7 x 2 = 83.4 A/H (theoretical full capacity)
That result would be excellent for a new, quality 90 A/H battery, realistic for 110 A/H, and decent for a worn but useful 130 A/H battery.
Best battery to replace the dud with (in my humble opinion) is a standard wet-cell battery designed for tail-lift trucks or buses / coaches. These applications are designed with tough plates to start a big engine, withstand vibration, but, most importantly, can take a long slow discharge. I used to pay 50p / A/H at my local truck battery specialist for my boat domestic batteries. Lead has got a bit more expensive since though.
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Topic: Battery
