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Battery Charging

  1. Jun 19, 2009 #1
    I have a red top optima battery like this in my truck:
    I don't drive this truck very often so it looses power after a while. I'd prefer not to disconnect my battery and loose the memory in various electronics. so i bought a small solar charger like this:

    It does not provide a sufficient charge to keep the battery alive.
    Is there a way i can measure/calculate (hopefully just with a multimeter) how much charge i need to keep my battery fully charged? I was thinking using some kind of calculation based on the voltage drop over a 24 hour period?

    If i only need two or three of these chargers then i might stick with the solar charger idea. If not, then i would have to resort to a trickle charger that plugs into the house. This would not be preferable since i don't park near the house.
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jun 19, 2009 #2


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    You'll have to disconnect the battery to measure the current that the accessories are requiring (use dc amp scale on your multimeter). Your charger appears to provide about 100mA (not much) at 12volts, on a good sunny day I imagine.

    So if your accessories are using more that 100mA, you'll need more solar cells.
  4. Jun 19, 2009 #3


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    That charger looks very small. If it did deliver 100 mA as Digoff estimates (and a fair guess) it might only average 20 mA over 24 hours allowing for no charging at night and not much in the morning or evening.

    Even so, a continuous current draw of 20 mA seems high for a truck. Maybe the truck battery is getting a bit lossy.

    The solar panel can draw current itself at night if it doesn't come with a series diode. This can be checked by applying an external 12 volts and measuring the current (if any) into the solar panel while it is in total darkness. If there is any current, a diode should be fitted.

    If you found you did need more charging current, maybe you could get a single much bigger solar panel and mount it on a post near where you park the truck. This would be better than a lot of small chargers which would be difficult to connect effectively.
    Or you could run a wire from the house to a charging point near the truck and charge from a power source in the house. It could be quite small wire as the voltage and current are low.

    You might be able to connect to the truck electrics via the trailer power outlet at the rear of the truck, depending whether this was switched.
    Just don't drive away with the wires connected.
  5. Jun 20, 2009 #4
    With the key off, you could connect a ammeter before disconnecting the battery terminal. This way, the memory stays intact.
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