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48v battery to power 24v bus

ArranP

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Is there anything wrong with wiring a 48v (16s) battery used in a 48v system ( 48v inverter / mppt charge ) to a 24v busbar... ie. connecting the negative of the 1st cell and the positive of the 8th cell to the 24v busbar, and again the negative of the 9th cell and the positive of the 16th cell to the 24v busbar ?

At the moment the design is to use a seperate battaries for each 24v and 48v busbars with a battery charger between so the 24v is charged from the 48v.

I was contemplating if that is indeed necessary and can the the 24v battery be eliminated by the wiring as described above.... of course I would need to put a 24v bms between the battery and the 24v bus bar.

Can anyone see a problem in doing this ?
 
your battery would quickly become unbalanced and unusable (or a nightmare to maintain).
both halves of the 48v battery would be connected to the 24v bus in parallel with same size/length cables... so the 24v load is spread evenly across both halves of the battery. Each BMS has cell balancing.
 
What do you mean "each" BMS? You have one 16S battery, how could you have more than one BMS?
The battery will be circa 50kw, There would be 3 BMS in total.

1x 24V BMS between cells 1 to 8 and the 24V bus
1x 24V BMS between cells 9 to 16 and the 24V bus
1x 48V BMS between cell 1 to 16 and the 48V bus

Some boat equipment ie. winches and windlass are 24v, where as the house system is powered by 48v inverter / mppt charger.

There will be 3x to 4x 5kw inverters connected in parallel total 15-20kw to provide power to the house load on the boat, i.e air-cons, fridge/freezer, tv, water maker, washing machine, hob, oven... usual household equipment.

However to DC equipment on the boat ie. electric windlass electric winches all operate on 24v.
 
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1x 24V BMS between cells 1 to 8 and the 24V bus
1x 24V BMS between cells 9 to 16 and the 24V bus
1x 48V BMS between cell 1 to 16 and the 48V bus
I'm gunna lose sleep thinking about this.

So thats half "2x 8S 24V batteries in series" and half 16S battery. Where did you get the idea to do this?
 
I'm pretty sure your eventual solution will need to be either 2x 8S batteries in series OR an 16S battery.

Then, a 48V to 24V step down converter.
This is where I started to use two a different set of 24v bateries for the 24v bus with a charger between the 24v and 48v. The electric winches are 2 to 3 kw not many buck converters can handle that draw...
 
I'm pretty sure your eventual solution will need to be either 2x 8S batteries in series OR an 16S battery.

Then, a 48V to 24V step down converter.
on reflection the 48v bms is not necessary as the 2x 24v bms will do the job...
2x 8s batteries ( 24v ) with 24V 0.5C BMS connected in parallel to the 24v bus and in series to the 48v bus.
 
I'm pretty sure your eventual solution will need to be either 2x 8S batteries in series OR an 16S battery.

Then, a 48V to 24V step down converter.

If within current capability, step-down converter would make use of off-the-shelf components.
Winch - can see where that should come directly from battery.

I had suggested +/-48V due to limited 96V equipment.
Just as some 12V batteries won't handle the voltage of four in series, 48V BMS may not like 96V.
I think a (big enough) clamping diode to keep either rail from being pulled past ground might address that. if one 48V BMS disconnects, the 96V motor gets 48V from the remaining BMS.

Having a 24V + 24V = 48V system would be possible the same way. Each 24V half would need its own charger, and might need to be protected from reverse-polarity on disconnect if not compatible with 48V series connection.

Possible risk with clamping diode - 24V could be harmful brownout to 48V appliances. Better to use batteries having BMS which tolerates the voltage, e.g. if build from 12V batteries that are OK 4 in series. If it disconnects, appliance is disconnected. Using a proper 8s BMS you'll have to dig deeper to make sure it can handle the voltage.

Maybe you would want a 48V to 24V charger, or 24V isolated to 24V, as an active balancer between the 24V portions. You would have to work out how to control that.
 
i upgraded from 24v lead acid to 48v lifepo4. I couldn't just dump the lead acids and had two small 24v inverters so i got a 15a 48v to 24v buck converter and set my system up to charge the 24v lead acid bank from the 48v lifepo4 bank, then ran my 24v inverters from that. Super cool as the 48v bank through the buck supplies juice to any load under 15a, and the 24v lead acid works as a supplemental power to provide 16-200a, restoring the used juice in a controlled way (max 15a) from the 48v bank. I just added a low voltage cutoff so now if my 48v bank gets low, i can disconnect the 24v lead acid until the voltage recovers in the 48v bank, effectively making the 24v bank a UPS for the small inverters. It has functioned flawlessly, created a way to maintain a critical load when the 48v bank is very low and allowed an independent supply to the 24v inverters for about 8 hours, allowing the 48v bank to recover more quickly. I'd definitely recommend this.
 
Each 24V half
If within current capability, step-down converter would make use of off-the-shelf components.
Winch - can see where that should come directly from battery.

I had suggested +/-48V due to limited 96V equipment.
Just as some 12V batteries won't handle the voltage of four in series, 48V BMS may not like 96V.
I think a (big enough) clamping diode to keep either rail from being pulled past ground might address that. if one 48V BMS disconnects, the 96V motor gets 48V from the remaining BMS.

Having a 24V + 24V = 48V system would be possible the same way. Each 24V half would need its own charger, and might need to be protected from reverse-polarity on disconnect if not compatible with 48V series connection.

Possible risk with clamping diode - 24V could be harmful brownout to 48V appliances. Better to use batteries having BMS which tolerates the voltage, e.g. if build from 12V batteries that are OK 4 in series. If it disconnects, appliance is disconnected. Using a proper 8s BMS you'll have to dig deeper to make sure it can handle the voltage.

Maybe you would want a 48V to 24V charger, or 24V isolated to 24V, as an active balancer between the 24V portions. You would have to work out how to control that.

Screenshot 2021-07-29 082602.pngThanks for your reply I have drawn up a diagram of what I am contemplating.... are there any safety measures I am missing ?
 
Thanks for your reply I have drawn up a diagram of what I am contemplating.... are there any safety measures I am missing ?
i upgraded from 24v lead acid to 48v lifepo4. I couldn't just dump the lead acids and had two small 24v inverters so i got a 15a 48v to 24v buck converter and set my system up to charge the 24v lead acid bank from the 48v lifepo4 bank, then ran my 24v inverters from that. Super cool as the 48v bank through the buck supplies juice to any load under 15a, and the 24v lead acid works as a supplemental power to provide 16-200a, restoring the used juice in a controlled way (max 15a) from the 48v bank. I just added a low voltage cutoff so now if my 48v bank gets low, i can disconnect the 24v lead acid until the voltage recovers in the 48v bank, effectively making the 24v bank a UPS for the small inverters. It has functioned flawlessly, created a way to maintain a critical load when the 48v bank is very low and allowed an independent supply to the 24v inverters for about 8 hours, allowing the 48v bank to recover more quickly. I'd definitely recommend this.
Or maybe it would be more safe to place the BMS to isolate the busbar instead of the battery ?Screenshot 2021-07-29 084751.png
 
I don't think you'll also have a 48V BMS.

BMS connects to each cell. Probably have to put its current shut and disconnect mechanism where that was designed to go.

It's going to have to be heavy for some loads, correct? Maybe you will have BMS control a relay. Then maybe either BMS can disconnect all loads, both 24V and 48V.

First thing to make sure of is that the 24V BMS can take having two in series. Some 12V batteries can be connected in series, some can't. I assume that is due to voltage applied across its FETs by the other battery.

Stuff connected to the upper 24V battery (maybe just a charge controller) could have metal parts referenced to 24V. See if chassis is isolated. Things like shields on data cables, etc. Don't want any of those to carrying current if they short to ground.
 
I'm pretty sure your eventual solution will need to be either 2x 8S batteries in series OR an 16S battery.

Then, a 48V to 24V step down converter.
Just do a step-down converter. What you're talking about will be fraught with maintenance issues and nightmares (if it even works). it's an interesting thought, but sounds scary to be playing with a lethal voltage in abnormal and experimental ways, like that.
 
Just do a step-down converter. What you're talking about will be fraught with maintenance issues and nightmares (if it even works). it's an interesting thought, but sounds scary to be playing with a lethal voltage in abnormal and experimental ways, like that.

A step-down convert would be my preferred option, however I have not ye seen any that can handle loads of 3000 watts.... the windlass and winches draw 3000 watts.
 
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