Battery life test

[Magaz97]

I want to test my batteries, to ascertain how long I can run my instruments etc, before recharging is necessary. If I switch on my usual instruments and leave them until they start to 'close down' will this harm my batteries? I have 3 batteries. 1 for engine starting and 2 in parallel for domestics etc. The batteries are all 90aH and are about 18 months old. They have never been fully discharged.

Is there a better way of testing my systems?

Sorry if this is a dumb question; electrics are not my strong point.

Thanks in advance.

 

[pvb]

As instruments usually shut themselves off when the voltage drops below, say, 10.5v, there's little risk of harming your batteries. Best to recharge them as soon as possible though.

 

[ghostlymoron]

I asked this question the other day. This is the helpful answer I received.

" Quote Originally Posted by ghostlymoron View Post
I have a collection of suspect batteries kicking around and would like to test their capacities and ascertain which are useful and which can be dumped. If I do a drop test with a small lamp taking say 1amp and measure the voltage every hour, how do I interpret the results to tell me the ahr capacity. Do I need to measure the current as well? Excuse my ignorance.
Reply:
Doing an accurate test is not easy (you need temperature control, current stabilisation etc.) but you can certainly get an idea.
Make sure the battery is soaked at 20-25 deg C and test in that sort of ambient temperature.
Aim for a load of 5% of claimed capacity (since capacities are most often on a 20hr basis.) - e.g a 60W headlamp bulb for a 12V 100Ah battery.
Take voltage and current readings from time to time, increasingly frequently as the voltage starts dropping quicker.
Terminate the test (and recharge immediately) when you are down to 10.8V (assuming a 12V battery).
Estimate your mean current from the readings and multiply by test duration.
You have a rough approximation to the capacity the battery is delivering.
In industrial applications you normally scrap if achieved capacity is below 80% of nominal - but some nasty cheapo leisure batteries probably barely achieve that when new!
I would probably scrap at 50%."

Not so easy if you can't remove them from the boat. Deliberately discharging them to a low level is not a good thing and could change your good batteries into scrap.

 

[Plevier]

Not so easy if you can't remove them from the boat. Deliberately discharging them to a low level is not a good thing and could change your good batteries into scrap.

Deep discharge damage is incremental. You won't kill the battery by doing it once in a while down to 10.5-11V. It is important to recharge promptly and thoroughly though.

 

[Magaz97]

OK, thanks for the info.

 

[ClaireDuet]

Is there a better way of testing my systems?

Can you take them out of the boat?

Electrics aren't my strong point either... I took mine to Halfords and left it with them while they did a test charge diagnostic thing. I got a printout, but basically they said it was b*ggered (and to be fair I kinda knew that already)