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12V to 12V trickle charger help

Discussion in 'Power Electronics' started by ies, Oct 28, 2020.

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  1. ies


    Feb 8, 2012
    First some background: We have an RV that has two separate 12V systems.

    The first is the normal chassis system that any van would have - 12V battery, alternator, engine, vehicle electronics, etc.

    The second system is a totally separate (other than common ground) "coach" system, with it's own alternator, 12V AGM battery, and 350W solar w/converter, plus a bank of LiFePO4 batteries and an inverter/charger. The Lithium batteries/inverter/charger setup can be turned on or off separately, but when turned on, they are all in parallel with the coach 12V system. The idea is that when camping we run off the Lithium batteries/inverter in place of a generator for 120V AC, and going down the road the 2nd alternator also charges the Lithium batteries (when turned on). Also when plugged in to shore power, the inverter/charger will charge the Lithium (and AGM) batteries and provide 120V AC.

    When sitting unused (winter), the chassis 12V system is off (like any other vehicle), the coach 12V is getting charge from the solar system, and the Lithium batteries are off. I'm not particularly happy with having the 12V AGM battery on charge all the time, but it does seem to work out OK.

    Now the Question: I would like to find a circuit that trickle charges from the 12V coach system (which in daylight is always around 13.9-14V due to the solar system). to the 12V chassis system. I'm looking for a trickle charger that's limited to under 1A (open to suggestions on an actual current limit).

    Can anyone suggest a circuit (or product if they're out there) that can do this? I'm looking to keep some charge in the chassis 12V battery since after a month or two parasitic loads eventually deplete the battery. Even if the circuit doesn't bring the 12V chassis system up to the full 13.9-14V that the coach system is at, it would still keep some charge in the chassis battery.

    Yes, I could just put a high power diode between the two 12V systems, but then when going down the road there could potentially be significant current "siphoned off" the coach system into the chassis system, and I want to prevent that. The <1A trickle charger isn't an issue, but I want to avoid more than that.

    Any help would be appreciated. Thank you.
  2. dave9


    Mar 5, 2017
    Measurements of the voltage of each and the current draw in each state could be useful, but if all you need is to limit current to trickle the chassis system (aka primary starter battery) then a schottky diode and series resistor should accomplish this. Measure the parasitic drain and size the resistor appropriately to slightly more than equal that, having to calculate some sort of average power since solar power input will fluctuate based on momentary sunlight.

    This also assumes that the solar panel has the capacity to keep up, it is far less current needed to keep an unloaded battery topped off, than to keep both it and one with parasitic drain topped off. However most vehicles in proper working order should have no more than a few tens of mA in their low power state (after some time out period if there is a battery saver type circuit that times out after some period, probably an hour or less with no trigger inputs to put it in a higher power state. Merely disconnecting and reconnecting the battery could be enough to put it in a higher power state).

    Another option might be to add a separate solar array and switch between them to use the larger of the two for the battery needing more charge at that time, or tie them both together to give a higher combined charging capacity for both batteries simultaneously.
  3. ies


    Feb 8, 2012
    If I understand correctly the charging current will change based on the chassis battery voltage level. So if I size the resistor for the desired current limit when the chassis battery is at 13V and the coach's battery 14V (and accounting for the voltage drop across the diode), then when the vehicle is started and the chassis battery voltage drops considerably (maybe 10V?), my current across the bridge will be significantly more. And if I size it the other way (to limit current during starting) then I'll get much less charge when the battery is close to full. And maybe that's a good thing.
  4. dave9


    Mar 5, 2017
    Your numbers are a bit off. Engine off, chassis battery should be near 12.6V. Coach battery too for the lead acid, no idea what Li-Ion you have.

    During vehicle starting, chassis battery will droop momentarily, the few seconds the starter is engaged, then rise to near 14.something volts from alternator. You should size it with this measured 14.(n) voltage in mind. You will always have less current when the battery is closer to full, ohms law, but still it is 14.(n) with engine running the alternator, minus the diode forward voltage drop (maybe 0.2V) vs 12.6-ish for the coach lead acid battery.
    Martaine2005 likes this.
  5. ChosunOne


    Jun 20, 2010
    The Ah rating of each battery is relevant. A larger battery will require a larger trickle charge to keep it topped off. I'm assuming that RVs, like cars & trucks, come with with a range of different size batteries; but what dave9 says is true: A trickle charge for any battery of the scale you're working with should be in the tens of mA range.

    I'm not familiar with the charging characteristics or resting voltage of lithium ferrophosphate batteries, but your lead-acid battery, with engine off, will actually measure somewhere between 13.5 to 14+V, Open Circuit Voltage, for a few hours at least after the engine is turned off. That voltage will slowly drop over the next few days as long as nothing is charging or draining, until the battery chemistry settles down. Then it'll meter more like dave9 mentioned, but the Lead-acids I work with, when fairly new, measure closer to 13V. And yes, age matters too.

    That 14+ Voltage right after engine-off is bluster: It isn't ready to actually deliver that much under load. If you put a small load on the battery while you meter it, the voltage will drop down to what it's actually ready to give you, somewhere near the 12.6V dave mentioned.
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