Correct float charge rate?

Submitted: Friday, Mar 05, 2010 at 09:34
ThreadID: 76569 Views:1982 Replies:2 FollowUps:2
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I have a 3 stage charger with useless instruction "manual". The charger looks identical to the one pictured in the "Battery Power" article on this site.
When it gets to the float stage there's a choice of 3: 13.2, 13.5 & 13.8. I assume that different battery types are meant to be "floated" at different rates. So what applies to what?
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Reply By: trainslux - Friday, Mar 05, 2010 at 09:49

Friday, Mar 05, 2010 at 09:49
What type of batteries do you have??

Standard nz70, or deep cycle etc.
Each battery will have its own charge/ float rates found on their makers websites.


AnswerID: 407345

Reply By: Battery Value Pty Ltd - Friday, Mar 05, 2010 at 09:56

Friday, Mar 05, 2010 at 09:56
Hello Moose,

the correct float voltage for lead acid batteries varies with temperature.
The aim of the float charging is to prevent sulfation and self discharge.
If the battery manufacturer's recommendations cannot be obtained, the following is a rule of thumb:

Generally, float voltage needs to be set at between 13.4 and 13.8V @ 25 degrees. -3.8mV/degree/cell.

Ageing AGM type batteries may require slightly higher, around 13.8 to 13.9V to prevent sulfation.
Gel is somewhere in the middle of the road, and flooded require the lowest float voltage (although it won't hurt if set slightly higher because any loss of electrolyte can be corrected).

Don't take the selected output voltage of your charger for granted, as the tolerances can vary quite a bit - verify with a voltmeter with a fully charged battery connected.

You can use the different float selections to compensate for temperature swings.

Best regards, Peter
AnswerID: 407346

Follow Up By: Moose - Friday, Mar 05, 2010 at 10:18

Friday, Mar 05, 2010 at 10:18
Thanks for that most informative reply Peter.
Mine are flooded so if I set at 13.5 it should be sweet (after checking the actual output).
As a point of interest - you said it was temperature related. So as the temp increases does the float voltage get increased or decreased? And at what temperature intervals?
FollowupID: 677260

Follow Up By: Battery Value Pty Ltd - Friday, Mar 05, 2010 at 10:27

Friday, Mar 05, 2010 at 10:27
If temp goes up, voltage has to come down.

Ideally the temperature compensation should be proportional, i.e. not stepped, or in intervals.
But you can skip the compensation for a narrow temperature band, say between 20 and 30 degrees. Outside this region, compensation becomes more important for good battery health.

Best regards, Peter
FollowupID: 677261

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