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Building 72v battery from 3x 24v units?

silverramp

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Jul 30, 2022
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I'm interested in building a small boat, and the electric motor I'm looking at can handle 48-72v nominal batteries. I could use 48v, but the higher voltage would offer some advantages.

Is it possible to use 3x 24v (or 6x 12v) batteries in series for a 72v setup? i have found 12v/100ah batteries that advertise up to a 48v serial connection. But I'm having trouble finding something that supports 72v.

My reasoning is that I'd like to be able to take the batteries off the boat, when not in use, and hook them up as 48v to a home solar/inverter system.
 
Some bms, like the jk bms, can handle up to 24 cells. That would get you up to 72v nominal.
Do you think that if I had three packs with their own 8-24s BMS, that I could run the batteries either as a 72v series or a 48v series? Given that the voltages are within the BMS range?
 
You can build three 72v packs from 3v cells, and put those in parallel. If you build from 12v or 24v batteries, how will you keep the batteries balanced with each other?
 
if you charge the 3x 24v banks separately that might help with balancing. A quick google search reveals many 72v LFP bms's .. also MakeSkyBlue make a 72v MPPT if that's what you're looking for
 
I suppose what I don't understand is how can off the shelf 12v and 24v batteries advertise that they work in configurations up to 4s/2s (48v)? For example these WattCycle batteries say just that, max of 48v series. If these are wired up in series, are these balanced while charging/discharging? Is a 72v capable BMS able to work in all three configuration of 1,2 or 3x in series?
 
I think the issue is the overall voltage of the components in the BMSs of the 24 volt batteries. They might not be able to handle 72 volts.
+1, I think he will have to find 24V battery who allow 4S connection (96V).
I'm interested in building a small boat, and the electric motor I'm looking at can handle 48-72v nominal batteries.
Sound like a bit of trouble simply to work at 72V instead of 48V. Why not select proper motor and/or propeller to have best efficiency and power at 48V?
I ask because for all the trouble to don't stay at 48V why don't you go directly at 96V?
 
I suppose what I don't understand is how can off the shelf 12v and 24v batteries advertise that they work in configurations up to 4s/2s (48v)? For example these WattCycle batteries say just that, max of 48v series. If these are wired up in series, are these balanced while charging/discharging?

Balancing between batteries is something your system has to do. With batteries in series, it is not possible for a battery to balance its cells with another battery's cells.

Is a 72v capable BMS able to work in all three configuration of 1,2 or 3x in series?
Depends. The jk bms is 8-24 cells, so it is 24-72v. It only works with 3v cells.

it won't help you with 12v batteries. You will need to find a bms that can balance four 12v batteries in series.

If you are going to the trouble of putting batteries in series, you might as well go all the way to individual 3v cells.
 
As @yabert mentioned, higher voltage may mean more prop RPMs and that may mean a change in pitch to stay efficient. It is not the same as getting more speed on an e bike by increasing the RPMs.
 
+1, I think he will have to find 24V battery who allow 4S connection (96V).

Sound like a bit of trouble simply to work at 72V instead of 48V. Why not select proper motor and/or propeller to have best efficiency and power at 48V?
I ask because for all the trouble to don't stay at 48V why don't you go directly at 96V?
As @yabert mentioned, higher voltage may mean more prop RPMs and that may mean a change in pitch to stay efficient. It is not the same as getting more speed on an e bike by increasing the RPMs.

I'm actually looking at an electric jet, check it out! 96V is too high, and I'd still have the problem of needing to have a two 48v battery banks that are ok with being put in series up to 96v. @48v I could use the 8kw or 15kw motors, but its marginally more money to use the 25kw motor, and for that you'd need 72v (according to the customer service contact at ma-3d). So while I definitely could go the 48v route, if I can do the 72v I'd love to figure a way to do that.

Balancing between batteries is something your system has to do. With batteries in series, it is not possible for a battery to balance its cells with another battery's cells.


Depends. The jk bms is 8-24 cells, so it is 24-72v. It only works with 3v cells.

it won't help you with 12v batteries. You will need to find a bms that can balance four 12v batteries in series.

If you are going to the trouble of putting batteries in series, you might as well go all the way to individual 3v cells.
So if I made 3x 8s batteries using the jk bms, could I put those 8s units in series for 16s or 24s use? I'm kinda confused what is the limiting factor of, for example, a wattcycle 12v battery that prevents it as being listed for higher than 48v series connection. And if balancing doesn't happen between batteries, how can they list these as being able to be connected in series?
 
I'm actually looking at an electric jet
Look a bit like a student project, but they are certainly cool. Thanks to share.
Considering your 25 kW goal and the peak amps of 550A, you can stop searching of the shelf 24V battery.
I think your best bet is to build a custom 72V battery. Then you can design it in 3 parts to have 72V or 3x 24V.
You can start with LFP cells or you can look at EV modules in 6S format (Ioniq 5/EV6 module in mind). EV module are high quality and powerful, but more dangerous if you don't know what you are doing.
 

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