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Valence batteries in series - do you really need an external BMS?

Rustyfrenchman

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May 28, 2020
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Hi All,

I have looked on the forum and followed several discussions on this but haven't reached a proper conclusion.

I'm looking for replacement batteries for my system as my old Lead ones are kaput. I'm looking at some Valence U27 12XP ones and realise they have an internal BMS. I want to wire 4 up in series (and parallel) to make 260ah at 24v.

The question is: Do I need an external BMS or will a battery balancer (equalizer) do the job if I control the voltage? I've got an old Victron Multiplus Phoenix 3000w 70a 24v and I'll have a 30a victron MPPT coming from a . 750w solar array.

Any help would be much appreciated!

s-l1600.jpg
 
Here is the page at Lithium Works.


From the information on their page, I would say that you do need the BMS unit. Without the BMS box, the battery does not actually have a way of disconnecting the system in case of over charge or discharge, even with just one module. The BMS unit controls external contactors for protection.
 
Hello folks....newbee here to the DIY solar RV set ups. I got my hands on one of these 12XP-138Ah batteries for my small RV. If I am reading this right I need to get an external BMS for this battery???? Is that true even if I am just running one of these batteries??? I do not have the need or space for any more then one.
 
I ran an electric '74 Dodge Dart for 8 years with lithium without a BMS and I now use the pack in my off-grid solar system. The answer is simple: get LiFE cells rather than LiPo and manually bottom balance them (take them all down to 2.8v and then charge them together. The nominal charge is 3.35v on my cells so I charge them only to 3.5. I have 5 strings of 8 cells in my "24" pack so I charge them to a max of 28.8v. The manufacturer says you can charge them to 4.25v so 3.6v is very safe. After 10 years of use all of the cells are perfectly balanced still. I check the cells individually at least every couple days - the issue is if one dies (has happened twice in 10 years) you need to know it. Easy to see when a cell dies - the voltage is wonky immediately. I have only had cells die on start up after long term storage - no idea why - won't charge. Probably frozen too long.

The reason I say use LiFe cells is because they are very stable. They die on over discharge but do not catch fire on over charge easily. I tested one by charging until it caught fire which happened at 14v for a single cell! Thus I never charge a cell above 3.6v.

Just my experience with lithium. My biggest issue was getting my MidNite solar system to just do what I wanted.

Also, only charge Li above freezing according to the manufacturer. This is the biggest issue in Canada.

NOTE - GET TO KNOW YOUR CELLS AND HOW THEY BEHAVE! EVERY LITHIUM IS DIFFERENT.

Gavin
 
The answer is simple: get LiFE cells rather than LiPo and manually bottom balance them (take them all down to 2.8v and then charge them together.
99% of us are parallel top balancing and that's what is recommended if using for an ESS system. If using in an EV application then bottom balancing is recommended.

The manufacturer says you can charge them to 4.25v so 3.6v is very safe.
This is what they said years ago. They have since changed it to 3.65 volts. If you know of any manufacturer that is still saying the cells can be charged to 3.65 volts then please post the spec sheet for those cells.

The reason I say use LiFe cells is because they are very stable. They die on over discharge but do not catch fire on over charge easily. I tested one by charging until it caught fire which happened at 14v for a single cell! Thus I never charge a cell above 3.6v.
There are various videos posted on YouTube showing the same types of testing. I don't recall the voltage. I have not seen any that caught fire except one and he punctured the cell in two places.

Also, only charge Li above freezing according to the manufacturer. This is the biggest issue in Canada.
Supposedly there are cells that can be charged in sub freezing temperatures. Winston claims their cells can be charged at -45C. I have not seen any testing to prove this will not result damage to the cell. The cells people are buying on this forum must be charged above freezing.

NOTE - GET TO KNOW YOUR CELLS AND HOW THEY BEHAVE! EVERY LITHIUM IS DIFFERENT.
That's great advice even if all cells have the same specs and are from the same manufacturer. It's exactly what I have done and the learning process is an ongoing one.

Regarding the BMS, everyone on this forum recommends using a BMS including Will Prowse. If a device fails in the system the BMS will protect the cells.
 
Random reply from moi, I'm new here. I knew I wasn't the only one trying to balance 2 or more batt in series. I went all the way to program arduino-esp32 so I could see each battery charge live over cloud-mobile app. (solar to cloud, get it?), only to go back to attaching LCd meters per battery just to see near-individual levels.
I got a couple of questions and yes if I should make a new thread I will, just wanted to tag along on this one.
1- If one battery shows 13.2v in series, and the other 15., which is the case right now (under a 4pm sunny sky and a german "phocos" 20-amp solar charger). -Is this an absolute, SOC reading, or is the 15v reading being pushed there because the other battery is not taking charge properly? I'm afraid the 15 level will damage this unit.
2- Since disconnecting isn't an option, and BMS won't solve it, is the best option to spend one battery while still in series? -I can do this via a 12v inverter hooked on the individual, higher battery, then use this inverter output.
Thanks!, and yeah I can't spend too much, else I'd drop the AGM batteries and update.
 
What you have is a charge balance issue. Too balancing the two batteries may help in the short term. You do this by individually charging each battery to full independently. Now they may drift apart as they discharge, but if they are in reasonably good condition, they should come back up to full charge together.

What you need for the long term is something like this.


It takes energy from the higher battery and transfers it to the lower one. This is a 4 channel unit for 48 volts using 12 volt batteries.
 
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