The MB3938 20A & MB3939 40A DC to DC chargers are intended to keep a 12V secondary/house battery automatically charged, by taking power from your vehicle primary 12V battery, or a 12V solar panel. They are identical other than their Amp ratings (20A/40A), which determine the max charge rate for the secondary battery.
They will only charge the secondary battery when the source power supply voltage is high enough, and will charge from solar in preference to the primary vehicle battery, even if the solar output is lower than the max rating of the charger, in order to minimise the load on your alternator.
Auto charging of the secondary battery will normally activate when the primary battery voltage is greater than or equal to 12.5V - connecting the ACC sense wire will allow it to start from as low as 11.5V though. The units will always stop charging when Solar input drops below 9.5V, or the vehicle primary battery/alternator voltage drops below 11.5V, so your vehicle battery will not be flattened by the charger!
Note: These chargers are only suitable for 12V battery systems, and solar panels outputting below 30V. If your secondary 12V battery is a pack made-up of multiple batteries wired in parallel, you should regularly check the individual battery voltages with a multimeter, and separately balance-charge the individual batteries if their voltages are not all the same.
Recommended setup
The plugs are shaped differently to prevent incorrect connections - you can only connect like-coloured plugs. Plugs to suit those supplied with the chargers: Grey: PT4425 / Red: PT4426 / Yellow: PT4543.
Only connect with wiring harnesses, connectors and isolation switches that are rated for at least the max current your charger can supply!
Check the polarity of your vehicle/solar/house battery connections against the charger's plugs - they are marked with + and - and the black wires are a common negative ground. If your solar panel does not have the grey connector, PS5122 can be used to convert Solar Panel MC4 plugs to the grey connector type - verify the wiring colours match up before you connect, and swap the pins at the PS5122 grey connector if they are opposite in polarity - see how here.
- Connect the Yellow plug to the primary vehicle battery, or battery wiring harness.
- Connect the Grey plug to your solar panel
- Connect the Red plug to the secondary / house battery.
- Connect the Blue wire to a vehicle wire that provides +12v only when the vehicle ignition is on.
- Press the Mode button to select the chemistry type of your secondary 12v->12.8v battery.
How they work in specific usage scenarios
These chargers are set-and-forget, however if you want more detail on their operation, see below.
Charging from primary vehicle battery only, no solar panel
Connected to Primary battery (PB), Secondary battery (SB) - no solar, and no wire to ACC
If PB Voltage is ~12.5V ±0.2V then the secondary battery will start charging until PB voltage has dropped to below 11.5V (while under load) or SB is fully charged then it will then auto stop.
Charging from solar panel only, no connection to primary vehicle battery
No Primary battery (PB) connected, only Secondary battery (SB), Solar (PV), and no wire to ACC
If PV voltage is above ~11.25V ± 0.2V solar charging of SB will start at the max power the solar panel can provide. Once solar charging is engaged, the solar LED will blink, and it will charge until PV voltage drops to 9.5V or SB is full.
Charging from vehicle battery or solar panel, but without connecting the ACC wire
Connected to Primary battery (PB), Secondary battery (SB), Solar panel (PV) and no wire to ACC)
If PV voltage is above ~11.25V ± 0.2V, solar charging of SB will start at the max power the solar panel can provide. Once solar charging is engaged, it will charge until PV voltage drops to 9.5V or SB is full. If PV is not available and PB Voltage is ~12.5V ±0.2V then the secondary battery will start charging until PB voltage has dropped to below 11.5V (while under load) or SB is fully charged then it will then auto stop. PV charging will override charging from PB and will auto switch to PV charging.
Charging from vehicle battery or solar panel, ACC wire connected to vehicle or switch
Connected to Primary battery (PB), Secondary battery (SB), Solar (PV), and ACC wired to ignition
With vehicle running (ACC wire = 10-16V) If PV voltage is above ~11.25V ± 0.2V solar charging of SB will start at the max power the solar panel can provide. Once solar charging is engaged, it will charge until PV voltage drops to 9.5V or SB is full. If PV is not available and ACC is 10V+, PB Voltage is ~11.5V then SB will start charging until PB voltage has dropped to below 11.5V (while under load) or SB is equal to PB. If ACC is 10V+, PB Voltage is 12.0+V then SB will start charging until PB voltage has dropped to below 11.5V (while under load) or SB is fully charged then it will then auto stop. PV charging will override charging from PB and will auto switch to PV charging.
With vehicle off (ACC wire = 0V) If PV voltage is above ~11.25V ± 0.2V solar charging of SB will start at the max power the solar panel can provide. Once solar charging is engaged, it will charge until PV voltage drops to 9.5V or SB is full. If PV is not available and PB Voltage is ~12.5V ±0.2V then the secondary battery will start charging until PB voltage has dropped to below 11.5V (while under load) or SB is fully charged then it will then auto stop. PV charging will override charging from PB and will auto switch to PV charging.
In this setup, you can put a switch in the ACC wire if you would like to prevent the charger from starting when the primary battery is between 11.5V and 12.5V. If switched off, it will only charge from the primary battery when it reaches 12.5V.
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