Charging 48v battery from 48v bank

[solvej]

I'm in the process of building battery packs of 18650 cells. Each pack is / will be 30p13s. That turned out to be a suitable for filling the stackable fire proof storage boxes. Now I got the idea that it would be convenient to be able to pull one pack out of the house bank, hook it up to an inverter and use it as a mobile "generator" for power tools. I'd like it to be a part of the house bank most of the time, just that I'd like to be able to pull it out those days I need it other places.

The challenge is, how do I reconnect this thing to the house bank? Most times when the mobile pack has been away it will return in discharged state. While this pack was away the rest of the house bank has been charged by the solar. Hence the voltage difference may be as much as 10V. Just connecting the cables directly I think would make a dangerously fast recharge. Hence there is a need for some kind of controller between the house bank and the mobile pack. What kind of controller should that be? Would a constant current circuit do the job? Or a current limiting circuit? Would a pwm motor speed controller do the job? When searching for "constant current module", lots of the hits are pwm motor controllers.
What makes this different from most charging circuits is that:
1) both the source power and the battery being charged will accept dangerously high current unless limited. Circuit must have a current limiting functionality.
2) the target battery voltage when charged is the same as the chargers input voltage - which is variable!

And what happens the day the mobile pack returns home without being used (still fully charged) on a cloudy day (house bank discharged), hence the voltage of the mobile pack is higher than the house bank?

My plan is that when the mobile pack has reached same voltage as the house bank, I will disconnect the charging circuit and connect the mobile pack directly to the house bank.

 

[NVS]

I was asking a similar question. I want to charge a 12v battery with a 24v battery. I was going to use a scc and connect the 24v battery to the pv side of the scc, then hook the 12v battery to it. The scc should limit the current and charge at the proper voltage.

 

[solvej]

SCC should work in your case because they are build for charging a battery from a higher voltage source. In my case, say the house bank is a 52V when I get home with the discharged mobile pack. An SCC would then be able to charge it to maybe 49V. That is not enough that I would like to reconnect the mobile pack directly to the bank. I could connect it through a resistor for a while, but that burns off quite a bit of energy.

How does a pwm charge controller differ from a pwm (constant current) motor speed controller?

 

[mesquito_creek]

Plug a bench charger into the house bank/inverter to bring the mobile battery to a matched voltage and then plug it into the house bank…

 

[solvej]

Is there really such thing as a 48v to 48v bench charger? I do not want to go the route 48Vdc - 220Vac - 48Vdc

 

[mesquito_creek]

Is there really such thing as a 48v to 48v bench charger? I do not want to go the route 48Vdc - 220Vac - 48Vdc

No there is not a 48v to 48v... I was suggesting 48Vdc - 120Vac - 48Vdc. You described a somewhat infrequent need to pull off one battery and go mobile and just needed a way to return it to the bank fully charged and using a charger from the battery bank to refill and match voltage of a single battery is simple and keeps the whole process off the grid. But you will need to wait for the sun...

 

[solvej]

Going dc to ac and back to dc sounds to stupid. Sure it works, but when something sounds stupid, I'll look for something better. Guess I'll just order a motor speed controller to see how it works (when it arrives in a year or two.)

 

[brb58]

There are parts to make yourself a 48V to 48V DC to DC converter but if this is just an occasional thing, I see no issue with the DC to AC to DC even though you waste some power. For that matter, you could just connect the two through a power resistor(also energy wasteful) and they would eventually balance if that was your goal.

Just so you know....a DC to DC converter is already DC to AC to DC....it's just not boosting it to 120VAC through a huge transformer.

 

[brb58]

I was asking a similar question. I want to charge a 12v battery with a 24v battery. I was going to use a scc and connect the 24v battery to the pv side of the scc, then hook the 12v battery to it. The scc should limit the current and charge at the proper voltage.

That is a very easy way with those voltages. Doing 48V to 48V is way harder

 

[solvej]

Doesn't a pwm constant current circuit do something similar to connecting the two batteries through a resistor, except it makes less heat?

 

[TorC]

I'm not sure if a PWM controller is fit for what may be the equivalent of a dead short. I suspect you'll need a buck/boost regulator to do it safely and get the batteries to full charge. At that, unless every watt counts, you're probably best off to just use a charger on your inverter.