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Old 03-09-2020, 12:53 AM   #1
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18650 Lithium Ion battery pack to replace house batteries.

Has anyone built a battery pack out of 18650 cell batteries to replace their house batteries? I know the voltage for the lithium ion batteries is higher than lead acid battery. The lithium batteries I have are 3.6 nominal 4.2vdc max. So 14.4 to 16.8vdc when 4 are used in series. I'm either going to replace the house batteries with these or I'm going to build a solar generator with them with a 2000 watt inverter and just plug the whole rv into it. We have a 30 amp service. I know I won't be able to run the ac but I can everything else. I will be using 4p with 100 cells each then connect them together in series to get the 14.4 nominal voltage. There would be four packs like this with 100 cells in each for 260 ah capacity.
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Old 03-09-2020, 01:27 AM   #2
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Originally Posted by nvs4602 View Post
Has anyone built a battery pack out of 18650 cell batteries to replace their house batteries? I know the voltage for the lithium ion batteries is higher than lead acid battery. The lithium batteries I have are 3.6 nominal 4.2vdc max. So 14.4 to 16.8vdc when 4 are used in series. I'm either going to replace the house batteries with these or I'm going to build a solar generator with them with a 2000 watt inverter and just plug the whole rv into it. We have a 30 amp service. I know I won't be able to run the ac but I can everything else. I will be using 4p with 100 cells each then connect them together in series to get the 14.4 nominal voltage. There would be four packs like this with 100 cells in each for 260 ah capacity.


Build? No. Buy? Yes. I picked up some surplus packs from an EV startup that didnít need them anymore that are essentially 2s/seventy something p 18650 packs. I have them bundled in 6s configuration running a 24V bank in my travel trailer with 1000W of solar and a Victron 3000 VA inverter. I bottom balance.

I kept my original 12V system intact and a new panel with a 30A breaker that feeds my old main panel, a 20 A breaker for my AC, and room for another (future) AC in my TT. My thinking is I only need the second AC a few hours per day when the sun is shining...

I can run EVERYTHING on a 30A circuit and can get the trailer cooled enough to sleep on battery power alone.

This gives me a little over 400amp-hrs @ 24V. Works great!
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Old 03-09-2020, 01:16 PM   #3
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Have you seen Will Prowse YouTube channel? https://www.youtube.com/user/errolprowse

He does DIY builds and tear downs of retail products. Seems knowledgeable on BMS products as well.

There's a lot to know with lithium systems!
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Old 03-09-2020, 01:40 PM   #4
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Bms boards are about 5 bucks. But you'll have to learn how to wire them. And you have to have a circuit that will cut your field first on your alternator if you charge off of it. An emergency disconnect under high current will blow the alternator for sure and maybe a good bit of electronics with it.
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Old 03-09-2020, 04:21 PM   #5
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Bms boards are about 5 bucks. But you'll have to learn how to wire them. And you have to have a circuit that will cut your field first on your alternator if you charge off of it. An emergency disconnect under high current will blow the alternator for sure and maybe a good bit of electronics with it.
What do you mean by the emergency disconnect blowing the alternator? Is this something unique to a custom built Li/BMS setup as opposed to the professionally built commercial LiFePo batteries with built in BMS? I'm assuming there are also different temperature management requirements along with fire dangers associated with what you're proposing.
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Old 03-09-2020, 09:48 PM   #6
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What do you mean by the emergency disconnect blowing the alternator? Is this something unique to a custom built Li/BMS setup as opposed to the professionally built commercial LiFePo batteries with built in BMS? I'm assuming there are also different temperature management requirements along with fire dangers associated with what you're proposing.
Any lithium system should have a high temp/ high current disconnect device. You cannot just cut the load from an alternator that is putting out high current. The field is chopped first, cutting the output, then the emergency system disconnects the batteries.
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Old 03-10-2020, 12:03 AM   #7
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Any lithium system should have a high temp/ high current disconnect device. You cannot just cut the load from an alternator that is putting out high current. The field is chopped first, cutting the output, then the emergency system disconnects the batteries.
You use a DC to DC charger to control the charge of the lithium ion batteries from the alternator. The BMS board for this size build will be 300 amps and cost around $200. You can also get a DC to DC charger for 12vdc to 24vdc which I think I'm leaning to. As I would like to add solar to the system as well. I will leave the house 12vdc system intact for lights and pump. But the 24vdc would run all the AC appliances. While running down the road off of the alternator. I'll be building my battery pack from 18650 cells where each cell is fused just like they do on Tesla electric cars. That way if on cell goes bad it will just blow the fuse and be disconnected from the rest of the pack.
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Old 03-11-2020, 01:37 AM   #8
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Curious where are sourcing the 18650s and what price & mah?

The thing I worry about with the 18650 cells is making a secure diy cell connection/enclosure for all those cells that would withstand the vibration of rig... I would worry a bunch of spot welded cells that would eventually short with one weld failing...

I like these... You might consider plug & play lithium cell as they are relatively KISS...

https://youtu.be/buNxclZixuU

As for a BMS... all seem to be cheap Chinese black boxes you really don’t know what’s going on... So I would tackle the issues a BMS addresses separately... Buy a good low voltage cutoff (Like Victron)... A nice battery monitor/balancer (Like ISDT BattGo BG-8S) and finally a way of shutting down charging at low temps... (theses are what you want a good BMS to do reliably)...
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Old 03-11-2020, 06:57 AM   #9
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I have built several 52 volt batteries for our electric bikes out of the batteries I can only get them to last about 2 years dont know how well it would work in rv
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Old 03-11-2020, 07:42 AM   #10
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If you want to build your own batteries, check out https://diysolarforum.com. There is a lot of good info there.
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Old 03-12-2020, 03:14 PM   #11
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Curious where are sourcing the 18650s and what price & mah?

The thing I worry about with the 18650 cells is making a secure diy cell connection/enclosure for all those cells that would withstand the vibration of rig... I would worry a bunch of spot welded cells that would eventually short with one weld failing...

I like these... You might consider plug & play lithium cell as they are relatively KISS...

https://youtu.be/buNxclZixuU

As for a BMS... all seem to be cheap Chinese black boxes you really don’t know what’s going on... So I would tackle the issues a BMS addresses separately... Buy a good low voltage cutoff (Like Victron)... A nice battery monitor/balancer (Like ISDT BattGo BG-8S) and finally a way of shutting down charging at low temps... (theses are what you want a good BMS to do reliably)...
I've been using http://www.batteryhookup.com
They also sell fused nickel strips https://batteryhookup.com/collection...el-nickel-fuse

I will strap them down inside of a Ridgid brand plastic tool box. I want this pack to be portable for other uses too.
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Old 03-12-2020, 03:28 PM   #12
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I have built several 52 volt batteries for our electric bikes out of the batteries I can only get them to last about 2 years dont know how well it would work in rv
What I've heard, you should only charge your 18650 batteries to 4.0 instead of the full 4.2 volts to make them last longer. When you build your battery do you test each cell to get the mAH of each cell, or do you just throw them in and weld them up? To build a balanced pack you need to match the mAH of each parallel pack before you put them together in series. This will also make your pack last longer. You should do cell level fusing so if 1 battery goes bad it won't kill your whole pack.
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Old 03-12-2020, 03:36 PM   #13
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What I've heard, you should only charge your 18650 batteries to 4.0 instead of the full 4.2 volts to make them last longer. When you build your battery do you test each cell to get the mAH of each cell, or do you just throw them in and weld them up? To build a balanced pack you need to match the mAH of each parallel pack before you put them together in series. This will also make your pack last longer. You should do cell level fusing so if 1 battery goes bad it won't kill your whole pack.


This is how I run mine: charge to 4.0 and discharge to 3.3 although I rarely get much below 3.8.
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Old 03-19-2020, 10:25 AM   #14
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People have done Tesla battery slab conversions. It entails 24v conversion and back to 12 convertor. It's a better upgrade for older build where an inverter was already on the shopping list.
There are a lot of 24v hybrid inverters out there where you can boost small genny power to start air conditioners. Like
https://www.altestore.com/store/inve...E#SCNCONSW4024
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