Navitron Renewable Energy and Sustainability Forum

Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: steve1717 on June 07, 2009, 03:42:40 PM

Title: second hand battery voltage
Post by: steve1717 on June 07, 2009, 03:42:40 PM
when buying second hand batterys is there any rule of thumb laws ..what to and what not to ??ie ..lets say old forklift and ups batterys . flooded lead ..sealed and ..the ones where you can top up yourself ..and 12v there a minimum voltage they should be showing ..age ?..fluid levels ...what should i be looking at .. that is crucial thanks .steve g

Title: Re: second hand battery voltage
Post by: G8GMU on June 07, 2009, 04:19:21 PM
 Second hand batteries can be something of a mine field.

Avoid the cheaper end of caravan leisure market with only a few hundred cycles at best.

I Have some 7 year old Hawker Powersafe batteries 105 Ah (87Ah at the 3 hour rate) which sit at 12.6 Volts & just keep going. This is on my PV system I have a leisure battery which has never been disharged beyond 30% of it's 85 AH and that sits at 12.8 Volts (not used for the PV power system)

You really need to be able to test the battery having had a full charge the rested for some hours at 3 and 12 hours for a more meaningful result. Then a discharge at about 10 amps for a minute to remove any surface charge.

I like to see at least the 100% value shown below as a minimum (O K some clever person may indicate you can't have more than 100% lets not go there)  the maximun theoretical voltage is 13.2 that is   2.2V per cell x 6 a battery in good condition should show at least 12.6.

The voltages below give the state of charge & to some degree the condition in terms of life
for a used batttery

% of full charge  for '12 Volt batteries' 
100% 12.7
90% 12.6
80% 12.5
70% 12.3
60% 21.2
50% 12.1
40% 12
30% 11.8
20% 11.7
10% 11.6

I Hope this helps

Title: Re: second hand battery voltage
Post by: Justme on June 07, 2009, 04:42:18 PM
Now I would say that this is closer to the SOC

100% 12.8
90% 12.7
80% 12.6
70% 12.5
60% 12.4
50% 12.3
40% 12.2
30% 12.1
20% 12.0
10% 11.9

For a lead acid bat.

BUT that does not tell you any thing about how good the bat is.

You could be sulphated up so that tha bat has lost 50% of its capacity & the 12.8 will be telling you that its FULL but its telling you the sulphated available capacity if fully charged not that the bat has a full charge of all its capacity.

Voltage alone is just a guide. You realy need a SG reading as well & better yet a few charge & discharge cycles under controlled conditions.



Title: Re: second hand battery voltage
Post by: desperate on June 07, 2009, 09:20:20 PM
when I have bought new car batts from my local shop the fella stick a hand held gizmo on the battery that draws a fairly hi current and at 6the same time measures the voltage, any help?

All I know is that battery acid eats my genes.     facepalm


Title: Re: second hand battery voltage
Post by: G8GMU on June 07, 2009, 09:35:56 PM
Both tables are pretty much right.

There are temperature considerations which have an influence on voltage. As you say voltage is only one indicator. The best & very expensive method of battery test is to use a heavy  dynamic load that pulls the voltage down over time, indicates that time & end voltage, then you read this off against the table for your particular battery. These units cost an arm & a leg, (no pun intended) a friend who owns a mobility center has one for testing powered wheelchair batteries I have had use of it, it really does the job.

I am sure the debate will rumble on, as we all have our own methods.

Title: Re: second hand battery voltage
Post by: EccentricAnomaly on June 07, 2009, 11:11:35 PM
All I know is that battery acid eats my genes.

I've had problems with chemicals eating my jeans but, thankfully, never my genes.

As to battery testing, isn't it pretty indicative to just put a reasonably heavy load (C/10 or C/5) on it and see how much the voltage sags?

Title: Re: second hand battery voltage
Post by: scott on July 02, 2009, 10:50:51 PM
second hand batteries are only as good as the weakest cell
and do not mix and match makes/ratings

better of buying new batteries with 10 or 20 years lifespan
in 2v 6v 12v to over 6000ah
also new GEL/AGM  deep cycle batteries are sealed for life
12V255AH 225.00 each 3000+ cycles