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Charge gel cell with car charger?

Started by Mike Cook February 24, 2013
A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which 
doesn't work. The 12vdc wall wart puts out only ~ 12.4v.

I charged the lead-acid gel cell using a car charger. The charge rate 
(selectable) was 0.5A. Afterward the trimmer seemed to work as new. 

Can the car charger be used to charge this battery? He can install a timer to 
limit charge time if that's something required.

Thanks.

The gel cell is 12v.

On a sunny day (Sun, 24 Feb 2013 00:06:18 -0800) it happened Mike Cook
<mcham@NOTyahoo.com> wrote in
<0001HW.CD4F09FA05B8251DB04179BF@news.eternal-september.org>:

>A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which >doesn't work. The 12vdc wall wart puts out only ~ 12.4v. > >I charged the lead-acid gel cell using a car charger. The charge rate >(selectable) was 0.5A. Afterward the trimmer seemed to work as new. > >Can the car charger be used to charge this battery? He can install a timer to >limit charge time if that's something required. > >Thanks.
Old trick was to put a car headlight or some other low voltage bulb (break light) in series as current limiter. Maybe modern car has only LEDs, dunno, :-)
Followups set to sci.electronics.repair .

In sci.electronics.components Mike Cook <mcham@notyahoo.com> wrote:
> Can the car charger be used to charge this battery?
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&#2013266096;F (20&#2013266096;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.
> He can install a timer to limit charge time if that's something > required.
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 http://www.ereplacementparts.com/ryobi-150r-41ae150f034-volt-battery-trimmer-parts-c-7931_15633_18531.html *2 http://www.power-sonic.com/images/powersonic/sla_batteries/ps_psg_series/12volt/PS-1270_11_Feb_21.pdf *3 http://www.mouser.com/ProductDetail/Power-Sonic/PSC-12800A-C/?qs=XATL/JQ9g1zerdAMdrMzKQ== Standard disclaimers apply; I don't get money or other consideration from any companies mentioned. Matt Roberds
On 02/24/2013 12:06 AM, Mike Cook wrote:
> A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which > doesn't work. The 12vdc wall wart puts out only ~ 12.4v. > > I charged the lead-acid gel cell using a car charger. The charge rate > (selectable) was 0.5A. Afterward the trimmer seemed to work as new. > > Can the car charger be used to charge this battery? He can install a timer to > limit charge time if that's something required. > > Thanks. >
You answered your own question. As long as the battery doesn't heat up on a big charge you're probably OK. Trickle chargers can do more harm than good. I'd find a smartcharger with a "lead-acid" setting.
On 02/24/2013 12:30 AM, Mike Cook wrote:
> The gel cell is 12v. >
Most lead acid batteries are ca. 13.6 Vdc If your car battery ever got down to 12 Vdc the car would have trouble starting. Lead acid cells are about 2.22 Vdc.
"dave"
> > You answered your own question. As long as the battery doesn't heat up on > a big charge you're probably OK.
** SLA or "gel cells" are prone to gassing when overcharged. The pressure builds up inside until something gives - then you have bits of battery and acid all over the place. Voltage & current limited charging is the only safe and sensible way. ... Phil
On 02/24/2013 05:14 AM, Phil Allison wrote:
> "dave" >> >> You answered your own question. As long as the battery doesn't heat up on >> a big charge you're probably OK. > > > ** SLA or "gel cells" are prone to gassing when overcharged. > > The pressure builds up inside until something gives - then you have bits > of battery and acid all over the place. > > Voltage & current limited charging is the only safe and sensible way. > > > > ... Phil > >
They have vents.
"dave"
>Phil Allison >> >>> You answered your own question. As long as the battery doesn't heat up >>> on >>> a big charge you're probably OK. >> >> >> ** SLA or "gel cells" are prone to gassing when overcharged. >> >> The pressure builds up inside until something gives - then you have >> bits >> of battery and acid all over the place. >> >> Voltage & current limited charging is the only safe and sensible way. >> > > They have vents.
** So you have no fucking idea what really happens. .... Phil
Mike Cook wrote:
> A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which > doesn't work. The 12vdc wall wart puts out only ~ 12.4v. > > I charged the lead-acid gel cell using a car charger. The charge rate > (selectable) was 0.5A. Afterward the trimmer seemed to work as new. > > Can the car charger be used to charge this battery? He can install a timer to > limit charge time if that's something required. > > Thanks. >
THe OEM charger most likely is unfiltered. Put a cap on the output but do not connect it to the battery and then do a voltage check. If you see a jump in voltage then this means the battery is on its last leg and you really shouldn't push it. A good test for this condition is to fully charge it with your auto charger and disconnect it and test the voltage a day later or few hours later to note the discharge. It is possible the charger is bad but I am going with a bad battery. You can always get a weak battery to appear good with an extra current source but the charge will dwindle. Jamie