On board charging for a 12v Trolling Motor Battery - No shore power

[Moonbeam]

The best time to charge the LiFePO4 battery is when the solar output is regulated. The excess power can be used charging the battery instead of being wasted. In this way the power is "free". There is no detrimental effect on the house bank or need for additional dedicated panels.

The OP has indicated that the house bank is usually "98% by about midday and in float/100% by about 2pm". The solar controller is regulating the power for a reasonable portion of the day so there should be enough of time for charging the LiFePO4 battery.

Yes... my ideal scenario would be to use the existing solar excess in the afternoon. Are you suggesting B2B charger or inverter route to achieve this?

I need to decide if I want to go the LifePO4 route and become pot committed to the Bison. I had hoped to have a few more goes with the Bison first, using the old lead acid battery, before deciding if it was worth further investment. But got stuck as I could not re-charge using my puny 150w inverter.

 

[Moonbeam]

We have had an electric outboard for 10 years now and almost always charge it from main domestic battery bank. With 400Ah lead acid batteries and 400W solar, then the typical charge of 500Wh every 3 or 4 days is easy to manage and barely noticeable as we always have excess which is normally used to heat a tank of water once batteries go to float after lunch.

Are you heating your HW tank via inverter from the battery bank? Could I achieve this on a 280Ah bank with my HW tank which requires 1200 watts using say a 2000w or 3000w inverter? Our 2000w genny currently heats the HW if we are on the hook for a few days, but would love to not have to run it.

Sorry... slight thread drift.

 

[noelex]

Yes... my ideal scenario would be to use the existing solar excess in the afternoon. Are you suggesting B2B charger or inverter route to achieve this?

I need to decide if I want to go the LifePO4 route and become pot committed to the Bison. I had hoped to have a few more goes with the Bison first, using the old lead acid battery, before deciding if it was worth further investment. But got stuck as I could not re-charge using my puny 150w inverter.

A battery to battery charger will be more efficient.

 

[B27]

We have had an electric outboard for 10 years now and almost always charge it from main domestic battery bank. With 400Ah lead acid batteries and 400W solar, then the typical charge of 500Wh every 3 or 4 days is easy to manage and barely noticeable as we always have excess which is normally used to heat a tank of water once batteries go to float after lunch.

That's good to know.
What kind of distance does 500Wh take you in the tender?

It's a bit more awkward on a smaller boat, we have 110Ah house battery and sail in the UK which is not always sunny.

 

[Moonbeam]

If you have excess solar energy I would not install a dedicated solar panel to charge the LiFePO4 battery. A DC to DC battery charger is the best solution.

Sorry, yes, you already said

 

[Moonbeam]

With a lead acid battery like you're currently using to power the Bison, surely you can just parallel it with the house bank? Use a pair of jump leads.

This will discharge the main bank a bit, but no problem because you're getting it back up o 100% every day, anyway. Maybe it'll take 2 or 3 days to get back up to 100% on the days you need to charge the Bison's battery.

You only need a B2B charger if you use a lithium battery for the Bison because it's a different battery chemistry and you need to be careful not to expose it to too high a voltage ((3.3v per cell??). But if the battery is the sea chemistry as you house bank then your SmartSolar is already calibrated for the right bulk/absorption/float voltage levels.

Not sure what kind of current would be drawn when charging but the spec for the BMV-700 says it handles 1000A anyway (seems like a lot!) so you can monitor how much you're discharging the main bank - just ensure the negative jump lead is connected to the other side of the BMV's shunt, not to the house battery itself.

Could this work? I did give this option some thought (although I would need a more elegant solution than jump leads across the floor etc ) but I thought it was bad form to have batteries charging off the same charger that are at un-equal levels of charge? Especially if the Bison battery was nearly empty and the house bank full? Also I thought connecting/disconnecting the Bison battery will upset the Victron MPPT charger unless it's shut down first while this is done. At least with a separate B2B charger like the Victron Orion, I can have a remote on/off switch for safe connecting/disconnecting.

 

[KompetentKrew]

Could this work? I did give this option some thought (although I would need a more elegant solution than jump leads across the floor etc ) but I thought it was bad form to have batteries charging off the same charger that are at un-equal levels of charge? Especially if the Bison battery was nearly empty and the house bank full? Also I thought connecting/disconnecting the Bison battery will upset the Victron MPPT charger unless it's shut down first while this is done. At least with a separate B2B charger like the Victron Orion, I can have a remote on/off switch for safe connecting/disconnecting.

Best practice would be to have a separate B2B charger, but I don't see that just paralleling them would be much different than using a split-charge diode.

You're right to say that there will be current flow from the house bank to the Bison's battery - I'm not sure how significant it would be if the batteries are imbalanced. Batteries have internal resistance, so it wouldn't just rip across - I haven't done the maths (I think the ham radio test has some questions about internal resistance?) but I wouldn't have thought it would be too much. I mean, you parallel two different batteries when you jump-start your car - for the moments before you crank the ignition, the more charged battery is charging the other one. And you could measure with your battery monitor how much current flows as you connect the jump leads.

I welcome criticism of my proposal from the other forum members, because I don't know for sure.

 

[RupertW]

That's good to know.
What kind of distance does 500Wh take you in the tender?

It's a bit more awkward on a smaller boat, we have 110Ah house battery and sail in the UK which is not always sunny.

It’s easy enough to increase the 110Ah (we had 200Ah in a 24 footer) but obviously no point unless you can charge it every day. Our 950Whr battery got a very heavy 3m rib with 4 people with luggage or shopping about 4 half mile trips in calm tideless water to get down to 50 percent. It does better in the last 5 years with an ultralight aluminium (think tinfoil but bizarrely still undamaged after multiple beachings) 2.6m.

 

[B27]

It’s easy enough to increase the 110Ah (we had 200Ah in a 24 footer) but obviously no point unless you can charge it every day. Our 950Whr battery got a very heavy 3m rib with 4 people with luggage or shopping about 4 half mile trips in calm tideless water to get down to 50 percent. It does better in the last 5 years with an ultralight aluminium (think tinfoil but bizarrely still undamaged after multiple beachings) 2.6m.

Actually, trying to get a third battery into my boat would be painful, and/or involve excessively long wiring.

I think our current tender mostly potters along at small throttle settings of a 2HP motor and we don't get through much petrol in a season.
Most of the time, I guess 200 or 300 watts of actual power is ample to beat the tide in a small inflatable.