Charge gel cell with car charger?

Probably. Do you know what the amp-hour capacity is of the battery that is in there now? A little Googling *1 suggests that the original 12 V battery might have been six Gates/Hawker/Enersys "Cyclon" 2 V cells in series; the replacement that one site sells looks suspiciously like the common or garden 12 V 7 amp-hour "gel cell", plus a couple of brackets to make it fit. If the battery capacity is around 6 or 7 amp-hours, then the 0.5 A charger will work OK, as long as you don't mind it taking

12 or more hours for a full charge.

One manufacturer of these batteries *2 says this is how you charge the

12 V, 7 Ah size:

"Limit initial current to 2.1A. Charge until battery voltage (under charge) reaches 14.4 to 14.7 volts at 68°F (20°C). Hold at 14.4 to 14.7 volts until current drops to under 70mA. Battery is fully charged under these conditions, and charger should be disconnected or switched to "float" voltage."

So... the 0.5 A car battery charger should be OK. The charger you have may not do the automatic float and shutoff - more below.

The voltage output by the car charger will be slightly different than what a sealed battery needs; this is probably not too important in this application, but may reduce battery life a little. If you want, you can get an "official" charger for sealed batteries for $35 or so; *3 has

0.8 A output and will work a little faster. Or, get one of the automatic car battery maintainers like a Deltran Battery Tender or Schumacher Battery Companion; these usually have 1.0 to 1.5 A outputs and will automagically switch to a low-current charge once the battery is full. These are easy to find at local stores.

If the charger doesn't have any kind of automatic shutoff, then this might not be a bad idea. The million dollar question is "how long is enough". The answer depends on how discharged the battery is. The simple answer (for a 7 amp-hour battery) is 7 amp-hours / 0.5 amps =

14 hours. But: the charge won't be 0.5 A for the whole time. It may start off that way but it will go down as the battery charges up, so a full charge will take longer.

At a guess, I would get one of those 12 hour spring wound timer switches (often used with attic fans) from the hardware store, and put it in an outlet box along with an outlet and a cord. Maybe set it for 2 or 3 hours the first time, then when it shuts off, measure the battery voltage to find out how full it is. If not full enough, repeat. After a few runs you will know what to set the timer to initially.

*1 *2 *3

Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.

Matt Roberds

My 6-volt Deltran Battery Tender Jr. got a severe beating and went in the trash after several failed attempts to charge my motorcycle's battery over the course of a year. It _never_ worked since purchase. And warranty service required payment of fees that approached the cost of the charger. Junk.

Until you load it.

Unloaded, an old xformer/rectifier battery charger always had significantly higher voltage without a battery on it. BIG ripple!

But once a battery is charged and even peaked out, that higher voltage peak can blow the damned things up, so all charging sessions had to be monitored to keep them from exploding. Then better chargers came out.

Now, I wouldn't be surprised if there wasn't one you could operate or observe with your iPad. There are Wi-Fi and/or bluetooth handheld meters now. That would be enough to know when to go shut it off.