building a multi-voltage battery.

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Sunrise

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Jan 13, 2019
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5
Hi, I have a new project coming up and I would like to go outside of the box a bit. I want to build a large battery for my bike using a 14s pack, and also be able to run my 24vdc to220vacinverter. I was thinking the best way to go about this would be to split the pack up and build two 7s systems and run them in parallel when I use the inverter, and switch to series to power the bike. then I started wondering if I could build a 14s pack with a 14s bms, and also overlay 2 7s bms's at the same time, running them in parallel to have a 24v source. I am crazy right? I have a feeling I am missing something, but wanted some backup from my friends here to tell me what I must be missing. I think it would be great to be able to charge using my 52v fast charger, and not have to worry about disconnecting a cable and dealing with all that mess.
 

ajw22

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Nov 16, 2018
Messages
641
Yes, you're crazy :)

The easy part is taking the 14s system, disabling the 14s BMS and splitting into 2x 7s systems to power the inverter.

The problems start when you try to reconnect them back to 14s, because now you potentially have cells at very different voltages. Worst case: one 7s is totally empty, other 7s is still full. If you then try to apply a charge, the first set would go overvoltage. If you tried to discharge, the 2nd set would go undervoltage. Of course, the 14s BMS will (hopefully) stop you from doing either, and sloooooowly balance out the charge, but that couldtake days~weeks.


If I were you, I'd build a big 14s and just get a 48V inverter.
If you absolutely need the 24V, build a small secondary battery, and a small 48V->24V buck converter (with current limiter, so it can't overheat)that keeps topping up the 24V battery.
 

daromer

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Oct 8, 2016
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5,659
Why not buy a 48V inverter instead.. Would be alot easier.
 

Crimp Daddy

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Feb 21, 2018
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962
I also second the 48v inverter... while what you are trying to do is possible, in practice it might get old real fast.
 

Sunrise

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Jan 13, 2019
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haha, yeah, I thought there had to be something about it that would be undesirable. the guys on the endless sphere forum, even after repeating my question in a few different ways to have good clarity on the matter, arrogantly told me it would be fine and nothing bad would happen. glad I came over here.

Not being one to give up on an idea easily though, what if I put a voltage/amperage readout on each pack to keep an eye on things, and before converting from one voltage to another, made sure to top off both packs to 100%? the 24vdc-220vac inverter is a 5kw unit, and was pretty spendy for my shallow pockets, so I am trying to keep using it if at all possible. I also use 24vdc-110vac 3500w and 1500w inverters which were about $1250 for all 3 units. as you can understand, it would really suck to not be able to use them for this project.
 

ajw22

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Joined
Nov 16, 2018
Messages
641
Sell the 24V equipment on Ebay, Gumtree, or whatever. Will of course not be easy to get good money unless you have a trustworthy account with a long history of trades and good reviews.
I buy & sell stuff every now and then, just to build a good history. It's basically like having a good credit score.
 

daromer

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Oct 8, 2016
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5,659
You can of course run a 48v battery and then take out 24v but it means extra electronics doing so. You can also reconfigure the battery back and forth between 24 and 48v but why go the extra mile doing that? :) I would say its easier to swap the gear out if its just 1 inverter.
 

Bubba

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May 9, 2018
Messages
300
daromer said:
You can of course run a 48v battery and then take out 24v but it means extra electronics doing so. You can also reconfigure the battery back and forth between 24 and 48v but why go the extra mile doing that? :) I would say its easier to swap the gear out if its just 1 inverter.
I can see an active balancer coming in handy for doing this between the 2 halves of the pack.
 

Sunrise

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Jan 13, 2019
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5
I cannot sell the 24v inverters because I use them also in my solar trailer for bigger jobs and going cross country to off grid places to power them up. so I would have to buy a full second set of 48v inverters. that just isn't going to happen. I really want to save the space in my trailer as well since I bring my bike with me on trips like this. it all needs to be interchangeable if humanly possible, which it sounds like it is.

so far it looks like the best option for this project and sticking to the efficiency and modularity I am looking for comes down to making 100% sure the voltage stays the same between packs, and that is pretty simple to do.

thanks for the advice gents, I will get to building as soon as the parts arrive.
 

hazSolar

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Joined
Aug 12, 2018
Messages
16
What you want will definitely work, just like you said you need to pay a little more attention to things. I've done a similar thing with 2x18V packs. With limited hardware I run my e-bike at 36V, but charge it with an 18V charger.
 
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