See also charging electronic devices.
Under Construction

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75
Source: Deep Cycle Battery FAQ

Charging 12 V backup batteries:
Most 12V gell cells can be charged at 13.8 V to their long term (eg. standby power mode) capacity. However, 14.2 - 14.5 will get you full charge.

Most Gell Cell battery experts would probably tell you 13.6 V is about optimum for float charging a Lead Acid gell cell. Going to 14V or higher tends to shorten their lifespan by warming them excessively so that they "dry out" more quickly.

The charging current does not exceed 5-10% of the rated battery capacity (eg 10AH would be 1 Amp charge). (Deep Cycle Battery FAQ says max. of C/20 = 5% for gel cells) Leave on the supply until the current goes to .010 amp. or so...

A cheap way to measure current (Amps). From a 13.8 V supply place a 10 -30 ohm 10 Watt resistor in series with the supply and battery. Do make sure with a voltmeter across the resistor (remember I = E/R) that . Also, do fuse the line between the battery and the supply (try a fuse equal to the battery capacity (10AH = 10 amps).
Source: (no longer available)

I recently fooled around with charging a 35 AH Power Patrol SLA1156 (SLA = Sealed Lead Acid)
Here are some notes:

  • My two chargers 1 amp for lawnmower and 2/10/60 amp for car batteries both showed only about 12 volts when not connected, but as soon as you connected them to a battery they jumped to 15.8 v
  • I checked amperage while charging and it started at 0.8 A but gradually went down to 0.4 A on an older battery and 0.3A on a newer one and stayed there.
  • The battery showed 15.0 V right after charging at 1 A for 12 hrs. but within 1 min was at 14.4, at 15 min was down to 14.1V, at 1 hr was 13.6 V
    in 5 hrs it was 13.2, in 24 hrs it 13.1, in 1 month it was 12.9
  • A year later after I'd used the battery for a while it charged to 14 v but in 24 hrs was down to 12.5 v.

LC1-12-1A - Leoch 12V / 1A SLA Battery Charger
Deltran Super Smart Battery Tender Junior 12V 750mA, charge plus maintenance. $29.99

Batteries in Home & Garden.
Batteries in the products page Charging at Deep Cycle FAQ
LC1-12-1A - Leoch 12V / 1A SLA Battery Charger
Charging the lead-acid battery - Battery University
12 V power for ham radio
The charge of the mobile phone brigade by Graeme Philipson
Hand Crank flashlights and Mobil Phone Chargers
Solar Panels
Power requirements
Surge Protectors
Uninterruptible power supply
CTEK Multi US 3300 12V Smart Battery Charger Review

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last updated 14 Apr 2008