Charging one battery from another using a Solar charge controller?

[Karottemkopf]

You don’t need much resistance if the batteries have similar voltages.
Initial surge current is not long-lasting if allowed to reach reasonably high value.

Lightbulbs are one good option as they increase in resistance as voltage/current goes up and act like sort of constant-current limiter. 10x 55W 12V car headlight bulbs cost 20 bucks or something like that and limit the current to 50amps or so.
Other creative options would be a long piece of wire/cable, 6 feet section of rebar rolled to coil etc

I like this DIY inductor idea?

 

[Karottemkopf]

If one is comfortable with being fully in charge of the situation and owning the consequences, even tightening terminal bolts differently to 'manage' the connection as shown in the video will 'work'.. but whatever you do, unless you are 100% confident it cannot become unsafe, i would make a connection that does not require you to unthread anything to abort the operation! Needs to be 'pull-apart' or 'knock-apart with the safety stick'.. And considering the main danger here is heat, i would probably just put on the gloves BEFORE you run into something being too hot to separate with your hands.

It could be taken as being safety-first or over-dramatic.. if you understand the situation well enough you will be in no danger. But sometimes our faith in ourselves is the most dangerous thing.

Yes, working on formula student car battery with 600V might have made me a bit too confident?

 

[32 volt boater]

I was thinking of buying a 36 volt lifepo4 because I could dump it into (use it to charge) my 32v battery bank and use it with my big 32v inverter.

But the money better invested in a dc refrigerator and high efficiency ac unit.

 

[Vigo]

There are lots of possible use cases for paralleling lithium and lead packs together. One which im currently experimenting with is connecting a smaller lithium pack to a larger lead pack only when the sun is down in such a way that it's primarily the lithium that 'cycles' overnight, to extend the life of the lead.

But you get into a lot of the stuff that's been discussed here, such as what kind of voltage difference is required to get energy to flow from one into the other.. And i agree with what was said that there are times when doing DC > AC > DC between the batteries is just the easiest/cheapest/most practical thing even if it's not technically the most efficient. I look at one of the advantages of lifepo4 as being that it gives you 'wiggle room' in other areas because it can operate in a wider range of soc/dod without being degraded much.

So in a scenario where the difference between doing dc-dc energy transfer vs dc to ac back to dc might mean the lead spends more time at low SoC due to conversion inefficiencies, the lithium can tolerate that just fine anyway, meaning i can use inverters and AC charging devices i already have for the experiments vs having to come up with very specific dc-dc conversion devices which are either rare, low watt capacity, or expensive..

 

[Leeds]

What would you recommend to transfer energy safely (and not too $$$) from one battery to another?

A quality set of automotive jumper cables

 

[pullin_gs]

Using a MPPT as a battery charger?

can I use a 75/15 as a 24v to 12v battery charger? both batteries are Lifepo4.

diysolarforum.com

In almost all cases, MPPT input voltage has to be notably higher than charge voltage.

....unless of course you have a "boosting" MPPT charge controller.
I use it for my ebikes. Panel voltage is around 30V, battery is 52V
But you are correct that most MPPT charge controllers must be higher voltage than charge.

 

[Risky Rob]

....unless of course you have a "boosting" MPPT charge controller.
I use it for my ebikes. Panel voltage is around 30V, battery is 52V
But you are correct that most MPPT charge controllers must be higher voltage than charge.

How do you set up the charge? I have a MPPT boost charge controller at 30V and a 52V ebike battery.

 

[pullin_gs]

How do you set up the charge? I have a MPPT boost charge controller at 30V and a 52V ebike battery.

With my particular CC I have to select battery voltage using a push-button configuration switch.
An LED display cycles through various lithium ion cutoff voltages (24V, 36V, 48V packs) and a custom feature which allows up to 60V (80v?) manual configuration. There is no setup for input voltage: It figures things out on its own.

Build Report: MPPT Solar Charging Station for Etrike

Works great. Charges 10S6P 18AH pack at rate of 2.5amps in full sun. I use it for both my Etrikes. Anchored 100W panel to south-facing side of garage. Grounded panel to steel pipe on gas meter using 12awg solid copper. Drilled 1/2" hole through wall for 12awg solid copper wire to charge...

endless-sphere.com

I've since updated it to a 250W panel and higher-voltage pack for fat-bike.

 

[Risky Rob]

With my particular CC I have to select battery voltage using a push-button configuration switch.
An LED display cycles through various lithium ion cutoff voltages (24V, 36V, 48V packs) and a custom feature which allows up to 60V (80v?) manual configuration. There is no setup for input voltage: It figures things out on its own.

Build Report: MPPT Solar Charging Station for Etrike

Works great. Charges 10S6P 18AH pack at rate of 2.5amps in full sun. I use it for both my Etrikes. Anchored 100W panel to south-facing side of garage. Grounded panel to steel pipe on gas meter using 12awg solid copper. Drilled 1/2" hole through wall for 12awg solid copper wire to charge...

endless-sphere.com

I've since updated it to a 250W panel and higher-voltage pack for fat-bike.

OK, it's a charge controller feature. My charge controller doesn't have that range. Pretty nice.

 

[Ktpart]

I used to do silly stuff like this and it didn't work. Due to mppt algorithm, and no current regulation on battery, it will push to absolute limit and typically shut down. Last time I tried this was battery to battery charging a Tesla 24V pack. Caused an over current safety trip.

I do not recommend trying this ever. Very dangerous. The PV input is designed for use with panels, not a battery.

I was wondering if you have tried using two charger controllers, hooking the donor battery to the first one #1 as would normally do and run a connection from # 1’s load terminal to the #2’s PV input and to receiving battery.

Would the load terminal regulate the current Down to #2 CC?

 

[Rednecktek]

Just to throw it out there, I've used cheap PWM's as battery maintainers many times for my ATV's, generators, and motorcycles. I just connect my main 12v battery (from my larger inverter & lighting system) to the Batt ports, then use the LOAD ports to an SAE plug on the machines. Even the cheapest PWM controllers can be set to turn the LOAD ports on and off at certain voltages so I set them to float at 12.6v and cut off at 11.8v. When the main battery bank gets below the 11.8 it stops charging the machines and when the sun comes back out weeks later and gets the main bank charged up the controller turns the bikes/ATV's/Generators back on. This usually works if the weather isn't so bad that the standby draw on the rest of the system doesn't kill the batteries all together... which has happened often.