Multiple Charging Sources - Avoiding overcharge


A friend has a sailing boat and has asked me if I can solve this problem. 

He has a 200Ah AGM leisure battery and wants to have 3 possible charging sources:

1. AC charger for when at shore
2. Alternator for when motor is running
3. Solar panels for when at sea

Each of these sources will need to supply maximum charging capacity to the battery. Thus around 30-40A each. However, as I know, you cant have them working at the same time, as this could potentially could put much more amperage into the battery than the maximum recommended (20% of C).

What would the solution be? Is there a way of only allowing one charging source at a time?



  • jonrjonr Solar Expert Posts: 1,384 ✭✭✭✭
    An error prone solution, but you can manually turn off #1 and #3.     As soon as the battery hits absorb, then the issue goes away (it becomes voltage limited). 

    Maybe someone produces an over-current alarm.   In theory, all of the sources could be changed to feed an acceptable voltage into the solar charge controller.

    I am available for custom hardware/firmware development

  • mcgivormcgivor Solar Expert Posts: 3,600 ✭✭✭✭✭✭
    Generally speaking, the current needed to charge a battery is determined by the battery voltage itself , as long as all 3 chargers are set to the same parameters it shouldn't be an issue, but with various sources, some programmable others not, the one which has the highest voltage will prevail as the master, if it exceeds the voltage of the others, the others will not pass current. There are limitations to the amount of current a battery can absorb 13% of Ah capacity or as much as 20% depending on the battery. Alternatively a selector switch could be employed to choose which has priority, having a sound understanding of how such a system would work is useful. At this point one can only offer general information due to lack of details.
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  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    As Mcgivor noted, current will be mitigated to some extent by the battery itself.

    It seems to me it's unlikely all 3 sources would be at max current at once anyway. The alternator won't have full output at idle, and I wouldn't think you'd be much above idle at the dock, for example.

    IMHO, the main risk with too much current would be overheating the battery, so making sure all sources have remote battery temperature sensors should largely solve the potential problem, and is better for proper charging charging anyway.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
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  • jonrjonr Solar Expert Posts: 1,384 ✭✭✭✭
    During bulk, the electronic current controllers (#1 and #3) will automatically exactly match their voltages with each contributing as much as they can supply to charging current.    During bulk, the battery does not limit itself (which is why manufacturers publish amp limits).  You can easily confirm with an ammeter.

    I am available for custom hardware/firmware development

  • scrubjaysnestscrubjaysnest Solar Expert Posts: 175 ✭✭✭
    I run 3 sources in the RV. When going down the road the alt is primary until solar gets high enough to over come. When on shore power the convertor is primary until the solar gets high enough to over come. They have all played together well.
  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    I'm using 4 charging sources.  3 solar charge controllers, and an Iota 75A charger.  All of these use battery voltage to determine the state of charge.  As  the voltage increases, the charge current decreases from each of the chargers.

    I ONLY see max charge current from all sources if the battery voltage is down, AND a heavy load is applied to the inverter.  As soon as the load is decreased, the battery voltage starts increasing, and the current quickly starts dropping off.

    Towards the end of a charge cycle, some chargers will indicate the batteries are fully charged, and only a float voltage is being applied, while others aren't quite there for another 20 minutes or so.  Absolutely zero issues with my setup.

    MorningStar says their solar charge controllers can be used in parallel without issue, as long as each controller has its own set of solar panels, which mine do.  Chargers won't backfeed each other.

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