Using a 280ah battery to charge a 100ah... what devise is needed?

[Terrapin]

I have built two batteries. One 280ah for mobile storage that can be used in the house or as additional storage for camping. And the second that will be used as a drop in replacement for the campers FLA I went with the more jar resistant fortune cells for this application. Space limitations are only allowing me to mount the smaller fortune cells in the battery rack on the tongue of the trailer. I can't even fit a group 29 box there but the fortune cells will fit in the smaller box that can be secured.

Eventually the batteries may be moved inside but the original wiring and battery location is going to stay for this season.

So I was wondering what the most economical and safe way to recharge the fortune battery from the bigger Lichen battery would be? dc to dc? would solar chargers like Mppt or pwm work? since the charging source is constant would there be a preference between the two? Is there some other way?

 

[time2roll]

Assuming both are 12 volts, I would make the effort to use them in parallel. If both are fully charged there should not be too much power exchanged as you connect. A pigtail on each battery with an Anderson connector should work fine.

 

[Terrapin]

Assuming both are 12 volts, I would make the effort to use them in parallel. If both are fully charged there should not be too much power exchanged as you connect. A pigtail on each battery with an Anderson connector should work fine.

I am working off the idea they will always be in very different states of charge. The Lichens always topped off as an independent solar generator that feeds the shore power cord by inverter at very rare high demand times and the fortune powering house usages daily.

 

[rickst29]

When you connect them, they will temporarily be in parallel. If the voltage will be significantly different when connecting the pair, then maybe add a small resistor (capable of handling high current and generating some heat) - to slightly reduce the rate of balancing flow, to avoid an excessive in-rush from the higher-charged battery exceeding the BMS limit of the less-charged battery.

A connection with smaller wires or using a jumper cable (which will have considerable resistance at each of the 4 clip ends) is also an option. An MPPT Solar charger is not an option, because the MPPT would need much higher voltage from the source battery. But either a PWM controller or a DC-->DC charger might provide the current-limiting capability which you desire, with slightly less loss of power than a naked resistor. I'd probably choose to go with PWM controller first, they cost less.

 

[Terrapin]

When you connect them, they will temporarily be in parallel. If the voltage will be significantly different when connecting the pair, then maybe add a small resistor (capable of handling high current and generating some heat) - to slightly reduce the rate of balancing flow, to avoid an excessive in-rush from the higher-charged battery exceeding the BMS limit of the less-charged battery.

A connection with smaller wires or using a jumper cable (which will have considerable resistance at each of the 4 clip ends) is also an option. An MPPT Solar charger is not an option, because the MPPT would need much higher voltage from the source battery. But either a PWM controller or a DC-->DC charger might provide the current-limiting capability which you desire, with slightly less loss of power than a naked resistor. I'd probably choose to go with PWM controller first, they cost less.

Interesting, Thanks. I did not know that the MPPT requires a higher source voltage. The resistor isn't as appealing as it seems it would create waste across the entire charge cycle.

I don't understand the difference in operation of either solar charger. But I think I have read that the MPPTs will allow greater charge with a wider variance of in current. That is not a factor off a battery so a PWM charger seems worth examining.

Can you tell me if there is a specific spec that I should look for in one if I intend to charge as above off a 4S LiFePO4 battery with an overkill 120A BMS? ....Or maybe even an actual recommendation on one?

I ask because I have components I bought that are a waste of money because I failed to ask this stuff in the past. I mean, they may get used somehow, but I certainly don't need them sitting around till then...

 

[Supervstech]

I think you are going to need a boost converter to charge from battery.

 

[Terrapin]

I think you are going to need a boost converter to charge from battery.

TY that is not on my radar yet. Off to the search window for me...

 

[mikefitz]

I'd probably choose to go with PWM controller first, they cost less.

That's OK because it you wont loose much when it fails. A PWM controller relies on not burning out its fet switches due to the current limiting effect of the solar panel. Connecting two similar voltage batteries via a solar controller of any type will not work.

A battery to battery charger would work providing the unit can be configured to operate over the range of voltages on the source battery, 13.4v to say 11v.

If you are using an Overkill 120A BMS then a battery to battery charger within that 120A range would be suitable. Charging at 0.5C on a 100Ah cell would seem OK.

The Renogy DCC50S battery to battery charger (set up in smart alternator mode) charges at 50A with the source battery greater than 12v, and will cut off at 11.5 volts

To avoid all this complexity why not install the batteries in parallel. Charge or discharge both independently to the same resting voltage then join. If there is a worry about different charge levels make the initial connection via a low resistance power resistor or a long length of cable, the voltages will quickly equalize then join permanently.

Mike

 

[curiouscarbon]

dear @Terrapin given that you want it to be one way from what i gather, a dc-dc converter sounds appropriate.

victron dc dc converter 12vdc to 12vdc 9amp isolated might be worth evaluating for your needs. the other models support different voltages of combinations of 12,24,48

i have a similar need and my research led me to that line dc dc converters. the output voltage is adjusted with a potentiometer on the bottom of the device. it takes a wide range input voltages and convert to that constant voltage. so maybe be careful to thoughtfully select that voltage.

cheers hope this helps



edit: changed picture size (took up too much space)

 

[Terrapin]

That's OK because it you wont loose much when it fails. A PWM controller relies on not burning out its fet switches due to the current limiting effect of the solar panel. Connecting two similar voltage batteries via a solar controller of any type will not work.



To avoid all this complexity why not install the batteries in parallel. Charge or discharge both independently to the same resting voltage then join. If there is a worry about different charge levels make the initial connection via a low resistance power resistor or a long length of cable, the voltages will quickly equalize then join permanently.

Mike

TY for those other ideas, I will look into the Renogy DCC505....Regarding this. Untill I can afford a second 280ah, that will full time in the camper after a rewire to a new location and support for the 2200w inverter and new converter I have to support the charging paralleling is on hold. Till then two batteries will not be mounted in the same vehicle at the same time. The portability of one is mandatory, it allows for a trolling motor battery power source in the fishing boat or along with the inverter as a solar generator for day tripping with the blender or other 120v conveniences. As a solar generator it will also be a home backup for grid failure essentials. Also for now only the full time fortune cells will fit in the trailer tongue battery box. When I add a thrid battery the fortune will go in the boat...ect ect ect... In short there are several factors at play and I have spent way more than intended. This is a last ditch effort to supplement one battery with the other.

 

[Terrapin]

dear @Terrapin given that you want it to be one way from what i gather, a dc-dc converter sounds appropriate.

victron dc dc converter 12vdc to 12vdc 9amp isolated might be worth evaluating for your needs. the other models support different voltages of combinations of 12,24,48

i have a similar need and my research led me to that line dc dc converters. the output voltage is adjusted with a potentiometer on the bottom of the device. it takes a wide range input voltages and convert to that constant voltage. so maybe be careful to thoughtfully select that voltage.

cheers hope this helps

View attachment 47288

Nice. I need to research this now... It seems very affordable to even go with a little higher charge rate

 

[Johnjayjrv]

Anyone know if the Victron DC to DC can be used to charge from a LifePo4 to Lead acid?

 

[mikefitz]

Victron DC to DC can be used to charge from a LifePo4 to Lead acid?

It will but ideally you want a dc to dc battery charger, like the Victron Orion Smart charger. This, unlike the simple dc to dc charger that will only generate a fixed output voltage, has battery charging profiles that will correctly charge the battery. For example, once the charge is complete it will drop to a float voltage to avoid over charging.

Orion-Tr Smart DC-DC Charger Isolated - Victron Energy

The Orion-Tr Smart DC-DC charger is a professional DC to DC adaptive 3-stage charger with built-in Bluetooth. For use in dual battery systems in...

www.victronenergy.com

The Orion is available with different conversion voltages and current ratings. Preset charge profiles or user settings. Switch on/off thresholds are user selective. Communicates via a Bluetooth phone app.

Mike