Want to charge a 24v LFP battery from a 48v system

RedDwarf

New Member
I'm running a 48v hybrid system to power my house (LiFePO4 500AH)

I have an additional 24v LFP battery I'd like to use for a subsystem in 1 room.
I'm no EE or electrician but I set up my home system myself (I can follow directions and do basic electronics things).

Can I charge my 24v battery from my 48v battery just using Buck converters to step it down?
Like just apply a constant 29v to the 24v or will I damage it doing things this way?
I have some good 48v buck converters on hand.

Thanks in advance!
 

rickst29

Solar Addict
I would not do that, because the 29v would never stop. You would have the battery sitting at more than 3.6 volts per cell, all the time. (100% state-of-charge is great for lead-acid batteries, but a bad idea for LFP. It hurts lifespan.)

I would instead buy an MPPT charger, programmable/tunable for LFP batteries. I would connect the 24v Battery as the "Battery", while connecting the 48V Battery set as the "PV array". "Boost" charging at maybe 28.5V (up to 3.56v per cell), and dropping into "float mode" at around 27.0 Volts (3.375 per cell), with BMS set to begin balancing at 3.36 or so. That's a lot more gentle on the battery and its BMS, in comparison to pushing it with 29V all the time.

If you do want to use a never-ending "CV" charge with the buck converter(s) you already have, to save the cost of buying a new MPPT Solar Charge Controller, then you might consider reducing the Voltage a bit - 28.4V (or even less) might cause a bit less "stress" within the target battery and BMS.
 

smoothJoey

SmooJo
I'm running a 48v hybrid system to power my house (LiFePO4 500AH)

I have an additional 24v LFP battery I'd like to use for a subsystem in 1 room.
I'm no EE or electrician but I set up my home system myself (I can follow directions and do basic electronics things).

Can I charge my 24v battery from my 48v battery just using Buck converters to step it down?
Like just apply a constant 29v to the 24v or will I damage it doing things this way?
I have some good 48v buck converters on hand.

Thanks in advance!
You don't say how much amperage you need.
The voltage drop at 48 volts is much less than 12 or 24 volts, could you just use a dc2dc converter instead of a battery + dc2dc charger?
 

rickst29

Solar Addict
You don't say how much amperage you need.
The voltage drop at 48 volts is much less than 12 or 24 volts, could you just use a dc2dc converter instead of a battery + dc2dc charger?
The DC->DC charges are generally Boost Converters, followed by 'Solar Charge Controller' voltage reducing battery chargers - all within the same box. But he doesn't need all that, because the 48v has absolutely no need of further "boost" before going into the SCC stage. He needs only the Voltage reduction and Battery Management of an SCC, So I think that a decent MPPT controller is the way to go.

But, as you described, the voltage drop between battery sets is much less if the "long wires" are high Voltage. The MPPT should be adjacent to the small set, unless safety concerns and installation limitations (e.g., lack of conduit) require that long wires be <30 volts, as required for low-voltage wiring in NEC.
 

smoothJoey

SmooJo
The DC->DC charges are generally Boost Converters, followed by 'Solar Charge Controller' voltage reducing battery chargers - all within the same box. But he doesn't need all that, because the 48v has absolutely no need of further "boost" before going into the SCC stage. He needs only the Voltage reduction and Battery Management of an SCC, So I think that a decent MPPT controller is the way to go.

But, as you described, the voltage drop between battery sets is much less if the "long wires" are high Voltage. The MPPT should be adjacent to the small set, unless safety concerns and installation limitations (e.g., lack of conduit) require that long wires be <30 volts, as required for low-voltage wiring in NEC.

I'm sure you know this but I will say it for those who may not.
The defining feature of a charger and the primary difference between a charger and a converter is charge termination.
 

RedDwarf

New Member
Thanks guys! Yes this was my concern; regarding the 29v that was just a ballpark figure for sure I could lower it to whatever makes sense.
I'll look into the MPPT controller solution; will the MPPT algorithm would gain anything over a cheaper PWM controller on a "constant" source like a 48v line in?
Would it make any sense to go for a PWM solar charge controller instead?

if I don't mind underutilizing the battery, could I just charge at the safe float voltage (27v) ?
@smoothJoey the 24v battery needs to be part of the solution because it's an extra 2600 whr I need (ok want) to have storing energy.
Thanks @smoothJoey and @rickst29 for helping me!
 

rickst29

Solar Addict
Thanks guys! Yes this was my concern; regarding the 29v that was just a ballpark figure for sure I could lower it to whatever makes sense.
I'll look into the MPPT controller solution; will the MPPT algorithm would gain anything over a cheaper PWM controller on a "constant" source like a 48v line in?
Would it make any sense to go for a PWM solar charge controller instead?
PWM would be not be a great choice for the case of the "one room" being significantly distant from the bigger battery. With MPPT, the "boosted" Voltage allows less current and less wire losses along the long path. PWM works by very rapidly switching the 'Solar connection' on and off, and cannot charge the target battery with anything more than the current which occurs on the long wire.

OTOH, if that current will be adequate for your needs - a PWM can be bought for less money. Just make sure that's easy to program properly for charging LFP batteries.
- - - SmoothJoey had great advice for a "float target" to be used, if you configure with any of these (MPPT, PWM, or boost converter alone).
 

RedDwarf

New Member
PWM would be not be a great choice for the case of the "one room" being significantly distant from the bigger battery. With MPPT, the "boosted" Voltage allows less current and less wire losses along the long path. PWM works by very rapidly switching the 'Solar connection' on and off, and cannot charge the target battery with anything more than the current which occurs on the long wire.

OTOH, if that current will be adequate for your needs - a PWM can be bought for less money. Just make sure that's easy to program properly for charging LFP batteries.
- - - SmoothJoey had great advice for a "float target" to be used, if you configure with any of these (MPPT, PWM, or boost converter alone).
Ah I understand now, thanks. If the MPPT will raise the voltage (to save on the voltage drop) then it probably needs to be on a separate line than other items drawing on my 48v battery right? I was planning to tap into an existing line.
Right now, I'm leaning towards going with the "just charge at float" idea.

Thanks a lot for explaining the MPPT benefit better; nice duh moment for me I never realized what MPPT algo was doing with changing voltages....
until now I kinda of realize a bit :)
 

rickst29

Solar Addict
Ah I understand now, thanks. If the MPPT will raise the voltage (to save on the voltage drop) then it probably needs to be on a separate line than other items drawing on my 48v battery right? I was planning to tap into an existing line.
Right now, I'm leaning towards going with the "just charge at float" idea.

Thanks a lot for explaining the MPPT benefit better; nice duh moment for me I never realized what MPPT algo was doing with changing voltages....
until now I kinda of realize a bit :)
Not exactly. An MPPT Solar Controller can only reduce voltage, turning more current. But 48V is plenty to start from, just put it on the 48V bus - along with other loads. Your "saved voltage drop" is running the long line at 48V, rather than running it around 27V. If you put the buck converter at the 24V end of the line, it would have the same benefit - but it would not adjust charge voltage according to the SOC of the 24 Volt battery.
 

toms

Solar Addict
I do a similar thing, except i charge a 48V remote bank from a bigger 48V bank.

I send 240Vac to the remote bank and use a 240Vac -> 48V dc charger.

The components were significantly cheaper than using all DC, and the efficiency is acceptable to me.
 

RedDwarf

New Member
Not exactly. An MPPT Solar Controller can only reduce voltage, turning more current. But 48V is plenty to start from, just put it on the 48V bus - along with other loads. Your "saved voltage drop" is running the long line at 48V, rather than running it around 27V. If you put the buck converter at the 24V end of the line, it would have the same benefit - but it would not adjust charge voltage according to the SOC of the 24 Volt battery.
Ah, I never intended to run a long line from the 24v; the 48v will run to the room where the 24v battery is; the battery will be near the laptop and other items it will power. Yeah voltage drop I get; I've spent a good amount of time doing calculations on cables and such to avoid this loss in the house as much as possible (it's the reason I went with 48v).
THanks again for all the input, spectacular community here I didn't expect so much help so fast!
 
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