Will Renogy DC-DC MPPT keep my starter battery topped up just from solar?


New Member
Jan 3, 2021
Hi all
I have a 2022 Volvo Penta D4 320 with the Volvo BMS for normal charging. I have a Renogy LifePO4 house battery and a lead acid starter. The Volvo BMS monitors status of both and does a decent job of it and alternator charging. The engine has a 150A alternator and will throttle back if temperature gets high - or so I'm told. I've been running the boat for a year so far and have no current concerns about that part of the system as it was set up this way and blessed by Volvo.

I have 200W of solar and currently have that connected directly to the house battery via a Victron MPPT charger. It works well. I often have need to run a freezer for weeks at a time from solar alone and this has worked without issue for the house battery with this setup.

However, this setup does leave the starter battery in a bad state where it can get low. I would like to charge the starter battery as a priority from solar, and then have the rest of the solar keep the house battery going. The Renogy Dual Input DC-DC On Board Battery Charger w/ MPPT seems on the surface to solve this problem (aside from only being able to have a single charge profile for either LA or LiFePO4)... but maybe only for alternator charging?

I'm concerned about how to interpret this from the manual: https://www.renogy.com/content/RBC3050D1S-G1/RBC3050D1S-Manual.pdf

Solar Panel Input
2. Connect solar panel, starting battery and service battery (Engine not running).
  1. 2.1 The solar input charges the service battery as priority. If the service battery voltage is
    lower than the boost voltage setting, solar panel will only charge the service battery.
  2. 2.2 If the service battery is in float charge stage, the starting battery will be charged at the same time. The charging voltage is limited at 13.8V. The charging amperage is limited at 25A.
  3. 2.3 After charging the starting battery for 1 minute, it will disconnect for 30 seconds and check the starting battery voltage. It will continue to charge starting battery if the voltage is lower than 12.7V and will stop charging if the voltage is higher than 13.2V.

If I set the battery type to LiFePO4, the manual says the charge profile does not have a float state... So when the manual says, "2.2 If the service battery is in float charge stage, the staring battery will be charged at the same time." ... Does that mean that it will never charge the starting battery, or will always charge it?

And if I set the battery type to Lead Acid, it looks like it will *only* charge the starter once the house battery has charged to 13.8V? For LiFePO4, that's a 100% charge, and often on solar days the battery is sitting significantly lower than 100%...
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Hmm. I guess I could set it up so that as far as the DC-DC charger is concerned, my starter battery is the house battery. That might work.

Renogy confirmed that effectively the solar will only charge the house battery.
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Also wondering if I can hook the current solar output to the Volvo BMS alternator in. It isn't clear from the manual/schematics whether it will work without the engine running.
I would just do a small Victron DC-DC charger from house battery to start battery. You only need a small "trickle charge" to keep the engine battery healthy. You can program it for the exact charge profile required by the LA battery.