I want to charge a deep cycle battery with a solar charge controller and dual battery isolator powered by the car alternator. The dual battery isolator will provide the most charging power when driving and the solar will help to keep the battery topped up when camping.

As I am feeding two power sources into one battery do I need diode oring to avoid damaging the two different chargers?

Below is a the wiring diagram of a 12V system. The Dual Battery Isolator will provide up to 120A and solar up to 10A. So I thought I put strong Schottky diodes in protect each charger.

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  • \$\begingroup\$ probably very inefficient what are your requirements for efficiency and panel specs. \$\endgroup\$ Jul 1, 2017 at 6:05
  • \$\begingroup\$ Hi. I'm new to this why and what is in inefficient? I have 60W panels on the roof. What I'm after is that the alternator does the charging each day and the panels keep it going once I stop driving. \$\endgroup\$
    – chriz
    Jul 1, 2017 at 11:29
  • \$\begingroup\$ Alternators use variable field currents to regulate current and thus efficiently optimize charge current. PV Panels however prefer impedance matched loads which is usually around 80% of the open circuit voltage and called the Maximum Power Point or MPT. A cheap MPT controllers is your wisest bet which is basically impedance matching the PV current source to a CV regulator with a voltage overcharge limiter by some design choice. Alternators already have 6 diodes, but the PV will need a proper reverse protection which can be a MOSFET switch. \$\endgroup\$ Jul 1, 2017 at 11:44
  • \$\begingroup\$ Thanks Tony. Just so that I get this right you are suggesting to replace my solar PWM with a MPT (aka MPPT) charge controller to increase the efficiency. But I can leave my dual battery isolator in place as per the diagram? \$\endgroup\$
    – chriz
    Jul 1, 2017 at 11:55
  • \$\begingroup\$ Wouldn't the MTP take care of the overcharge limiter and the reverse protection? \$\endgroup\$
    – chriz
    Jul 1, 2017 at 11:58

1 Answer 1


The Schottky diodes are probably not required the battery isolator already is a diode or the equivalent of a diode and the output of the solar charger can handle the charging voltage as provided by the alternator.

The solar part of the system should already have a diode in it somewhere to prevent reverse flow into the panel.

The larger problem is that 'smart' solar chargers sometimes do not play well with other charging sources, they get confused by the voltage changes caused in your case by the alternator. This will not cause damage but could cause the solar charger to operate at less than its peak performance. You will need to check the specifications for your charger. if it is a 'dumb' pwm controller without a 3 stage charge algorithm you should be ok.

  • \$\begingroup\$ Hi thank you for your reply RoyC. It's a cheapish PWM charger from a no-name brand from China. The manual states that it has a buck charge at 14.2V and a float charge 13.7V mode; both are adjustable. It also features a dual MOSFET reverse current protection. Does make it 'smart' and if so what are my options? \$\endgroup\$
    – chriz
    Jul 1, 2017 at 11:19
  • \$\begingroup\$ What causes the switch from bulk charge to float charge? It is this mechanism that is likely to get confused. \$\endgroup\$
    – RoyC
    Jul 1, 2017 at 14:43
  • \$\begingroup\$ I have to pass on that because it does not say in the manual. Would a MPT have the same problem of 'getting confused' as a PWM? \$\endgroup\$
    – chriz
    Jul 1, 2017 at 20:24

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