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Old 9th Aug 2020, 11:50 pm   #1
Bazz4CQJ
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Default "Battery Manager" for a Li ion Battery

I’m about to assemble a battery using 3 lithium ion cells (type 18650). This will give me around 12V, and be used in various roles around the house, garage and garden where I currently use a small 12V SLA.

I’ve got very limited experience of using these cells and I considered what I was going to do about charging this battery, spotted some modules (photo attcahed) , widely available on-line, purposely made for a “Battery Management Systems” (BMS) for three Li-ion cells. The module is fed with ~12V and each of the cells is charged “separately”. Similarly, some aspects of their discharge are managed by the module.

However, it seems that I may have made optimistic assumptions about the extent to which these modules do their managing (it’s not very specific in the advertisements) and there are some Youtube videos suggesting these modules could be a waste of time, or worse.

So I’m presently thinking about best to assemble this battery and just wondering if any one else has bought and used any of these BMS boards, or alternatively, can tell me what an ideal BMS would actually do?

Thanks
B
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Old 10th Aug 2020, 3:04 am   #2
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Default Re: "Battery Manager" for a Li ion Battery

After looking a lots of ads, I finally found this more detailed spec;

Features;
Short circuit protection. Over current protection.
Overcharge protection. Over discharge protection.
Suitable for all kinds of 3.7V lithium cells.

Applied to 3 cells in series (3S):
10.8V (rated voltage of polymer battery)
11.1V (18650 or 3.7V rated voltage of lithium battery)
12.6V (full charging voltage of 3S lithium battery)

Maximum discharge current: 20A
Overcharge detection voltage: 4.25-4.35V 0.05V
Over discharge detection voltage: 2.3-3.0V 0.05V
Maximum working current: 10-16A
Transient current: 18-200A
Working temperature: - 40-50 ?
Storage temperature: - 40-80 ?
Static current: < 6uA
Internal resistance: < 60mO
Service life: > 30000 hours.

That sounds reasonable if it's right I don't plan to have it deliver more than 4-5 amps steady current.

B
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Old 10th Aug 2020, 9:25 am   #3
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Default Re: "Battery Manager" for a Li ion Battery

dont suppose you would have link to amazon for this
regards
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Old 10th Aug 2020, 9:53 am   #4
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Default Re: "Battery Manager" for a Li ion Battery

That looks like a fairly standard 'battery management system' (BMS) for Li-ion cells. All it will do is prevent damage to the cells by overcharging or overdischarging, and, looking at the PCB, it looks like it has some attempt at balancing the fully-charged voltage between the cells.

It'll do the job, but it doesn't include any charge regulation circuitry - you're essentially dealing with the naked cells. You need an external charger which will limit the charging current, probably to around 2A for 18650 cells, and detect when the fully-charged terminal voltage (12.6V in this case) is reached, then shut off once the current falls to a reasonably low value. Good chargers also detect undervoltage and gently bring the cells up to their normal operating range, before cranking up the charge current to charge them quickly.

If you have no other power management in this application, the BMS should be set to cut off discharge at a cell voltage of 3.0 V for Li-ion cells. Any lower than that starts to risk damage to the cells. The spec says 2.3-3.0V which is rather a wide range, so presumably it's configurable or you can choose it when you buy the module.

A side note on balancing: all cells, even nominally identical ones, are slightly different, so when charged in series, some will reach full charge before the others. A good BMS will then start to bypass those cells, shunting the charge current they would otherwise be receiving, so the remaining cells in the series chain can quite rapidly come up to full charge as well. This needs big, hot load resistors or cunning circuitry to shunt the current aside. I think this simple BMS has neither, only very small bleed resistors to provide some limited balancing once charging has completed. That means that as one cell reaches the cutoff voltage, the BMS will likely shut off the connection to the charger altogether and only reconnect it once the bleed resistors have taken the edge off the full charge on that cell. Then it'll reconnect the charger and the cycle will repeat until all the cells are charged. This can take a long time if there's a significant mismatch between the cells, and the on/off cycling can thoroughly confuse some chargers.

Chris
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Old 10th Aug 2020, 10:16 am   #5
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Default Re: "Battery Manager" for a Li ion Battery

Chris has said it all really.

You don't actually need output protection if you are prepared to use the cells carefully. This means recharging them well before they are fully drained, which is easy in some applications but more difficult in others. 18650s are pretty cheap, especially if you scavenge them from scrap battery packs, so the cost of occasionally messing things up and wrecking a few cells isn't great.

I use 2 18650s in my bathroom radio without protection and have never had any problems. I recharge them a couple of times a year.
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Old 10th Aug 2020, 10:43 am   #6
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Default Re: "Battery Manager" for a Li ion Battery

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Originally Posted by paulsherwin View Post
You don't actually need output protection if you are prepared to use the cells carefully. This means recharging them well before they are fully drained, which is easy in some applications but more difficult in others. 18650s are pretty cheap, especially if you scavenge them from scrap battery packs, so the cost of occasionally messing things up and wrecking a few cells isn't great.
This is true. Due to experience in the day job I may have a tendency to over-engineer battery management systems! One system I designed is rated to discharge its 100Ah battery pack in about 20 minutes and still has to deliver a very long service life (thousands of cycles).

For casual use, there's no problem about just being a bit careful. There is the risk that a seriously over-discharged cell can develop internal short-circuits and thus present a fire or explosion risk when recharged. The mechanism for this is that copper from one electrode starts to migrate through the cell towards the other, aluminium, electrode, which can create a conductive path which is clearly bad news.

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Old 10th Aug 2020, 2:00 pm   #7
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Default Re: "Battery Manager" for a Li ion Battery

Firstly, for Mervyn, my actual boards came from ebay https://www.ebay.co.uk/itm/3S-4-5-20...72.m2749.l2649, but there are dozens of different sellers.

My plan is to charge the pack from a Coutant lab PSU, so both the max voltage and current can be preset. I also plan to include a small LED voltmeter on the battery with a "push to test switch" to moitor discharge level and probably a fuse of ~10 or 15amps? So I guess that all I'd really like to the board to do is maintain balanced charging?

B
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Old 10th Aug 2020, 3:49 pm   #8
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Default Re: "Battery Manager" for a Li ion Battery

You should definitely fuse it. These cells can deliver a fearsome current into a fault or short circuit, which can cause fires.

However, if you are using a proper battery controller then that should prevent overcurrent. A physical fuse is then just there to provide belt-and-braces safety insurance.
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Old 12th Aug 2020, 2:09 am   #9
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Default Re: "Battery Manager" for a Li ion Battery

What 'per-cell' charging voltage would you use for series charging these batteries? Same voltage as final battery voltage, or a bit higher? I suppose I'm asking what the DC input to the BMS should be (e.g. for the four cell BMS). The adverts for those Chinese ones make no mention at all of this.

Mike
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Old 12th Aug 2020, 3:43 am   #10
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Default Re: "Battery Manager" for a Li ion Battery

Based on the info in post #2, I'd plan to set the lab PSU at an off load limit of a tad over 12.6V, say 12.8V? I need to go away and check what the max current setting on the psu should be? The RS website has detailed specs for the Samsung 18650.

B
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Old 12th Aug 2020, 10:29 am   #11
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Default Re: "Battery Manager" for a Li ion Battery

Simple commercial chargers charge each cell with a fixed 4.2V voltage and some limit on peak charging current. Anything with a proper charge controller chip will vary the charging current as the cell charges, which is a bit kinder to the cell and crams a bit more charge in.

I take my cells out for charging and charge them individually using a charge controller, but that isn't practical if you need to charge them frequently.
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Old 12th Aug 2020, 3:19 pm   #12
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Default Re: "Battery Manager" for a Li ion Battery

My 3-cell 12V battery will have the cells permanently wired in, and that's why I was hoping that the BMS would take care of optimising the use of each cell.

The design team has handed over the drawings to production, so I should have it for testing in a week or three .

B
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Old 12th Aug 2020, 3:37 pm   #13
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Default Re: "Battery Manager" for a Li ion Battery

Quote:
Originally Posted by Bazz4CQJ View Post
My 3-cell 12V battery will have the cells permanently wired in, and that's why I was hoping that the BMS would take care of optimising the use of each cell.

The design team has handed over the drawings to production, so I should have it for testing in a week or three .

B
I was wondering about that -the optimising of charging for each cell. What happens when one cell is fully charged, but the others need to continue? Surely this would require bypassing of the charged cell, and a reduction of charging voltage to the remaining ones. That's why I asked about the (fixed?) input voltage to the BMS - with one or two batteries bypassed but the same input voltage and a lower charging voltage there's going to be a fair bit of power (= heat) to be dissipated somewhere, and I can't see any provision for that on those BMS boards. Maybe they use some sort of switching regulation, but they don't look as though they have that. I think the very least needed is to know the appropriate input voltage(s).

But, none of this is my "special subject"!

Mike
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Old 12th Aug 2020, 4:38 pm   #14
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Default Re: "Battery Manager" for a Li ion Battery

Quote:
Originally Posted by Boulevardier View Post
Quote:
Originally Posted by Bazz4CQJ View Post
My 3-cell 12V battery will have the cells permanently wired in, and that's why I was hoping that the BMS would take care of optimising the use of each cell.

The design team has handed over the drawings to production, so I should have it for testing in a week or three .

B
I was wondering about that -the optimising of charging for each cell. What happens when one cell is fully charged, but the others need to continue? Surely this would require bypassing of the charged cell, and a reduction of charging voltage to the remaining ones.
Yes, good balance charging does need this, and the board in the original post doesn't seem to support it. Lots and lots of simple Li-ion BMSes just don't bother, and terminate the charge cycle when the first cell is 'full', so they don't get the maximum capacity and cycle life from the cells. They just assume that the cells are closely-enough matched not to worry about it. I have a 'dead' Dyson battery pack in which the individual cells appear to be OK but their states of charge are really unbalanced: one cell always charges first and the BMS gives up long before the others are charged. This is after hundreds of cycles, though. One day I'll manually balance it and it'll probably be OK for a few hundred more!

There are various ways of doing 'proper' balancing, getting every cell to full charge. You can charge each cell individually from its own supply, which is inconvenient if they're in a series pack. You can have a BMS with a reasonably beefy bypass capability, so it can dissipate most of the charge current for those cells which are getting full. Or you can have a BMS which interacts with the charger, telling it to reduce the overall charge current once bypass starts on one or more cells, so those cells are guaranteed not to get overcharged and the others can slowly catch up. Or, there are some really clever (too clever, in my experience) BMS chips out there which work out each cell capacity during discharge, then try to bleed a small current from those with lower capacities during charge, so they all stay pretty much in sync all the way through charging and should reach full capacity at the same time. It's possible, though unlikely, that this module contains one of those.

Chris
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Old 12th Aug 2020, 5:20 pm   #15
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Default Re: "Battery Manager" for a Li ion Battery

I've just re-watched the Youtube vid which I referred to in post#1.https://www.youtube.com/watch?v=w8xtSTspFCE It's about 25 minutes long, but the conclusions are in the last 2 minutes, where he suggests that there is no balancing and the name "Battery Manager" is a bit optimistic.

Sadly, I've not spotted anything on the market that would do better, unless you go down the one charger per cell route.

B
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Old 12th Aug 2020, 5:43 pm   #16
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Default Re: "Battery Manager" for a Li ion Battery

For casual use you can worry too much about these things (and spend lots of money too). Chris's experience is with designing professional equipment where the battery pack must go through many hundreds of charge cycles and support very demanding applications with near 100% reliability. For home use this is just 'nice to have'.
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Old 12th Aug 2020, 6:01 pm   #17
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Default Re: "Battery Manager" for a Li ion Battery

Quote:
Originally Posted by Bazz4CQJ View Post
I've just re-watched the Youtube vid which I referred to in post#1.https://www.youtube.com/watch?v=w8xtSTspFCE It's about 25 minutes long, but the conclusions are in the last 2 minutes, where he suggests that there is no balancing and the name "Battery Manager" is a bit optimistic.

Sadly, I've not spotted anything on the market that would do better, unless you go down the one charger per cell route.

B
Looks like it is Chris's first, "simple" type then (and like his Dyson), - monitors V of all cells, but just stops charging all cells completely when just one has reached specified voltage.

Mike
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Old 12th Aug 2020, 8:18 pm   #18
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Default Re: "Battery Manager" for a Li ion Battery

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For casual use you can worry too much about these things (and spend lots of money too).
Well said. To get maximum performance out of batteries in difficult conditions needs a good BMS, and it's hard to get right.

For most domestic applications, a simple BMS like the one under discussion will prevent damage to the cells and avoid dangerous situations, and that's good enough. If you're prepared to replace the battery or intervene with some manual 'management' when it needs it, you'll be fine.

Chris
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Old 12th Aug 2020, 9:16 pm   #19
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Default Re: "Battery Manager" for a Li ion Battery

This is my first foray into Li ions, and I've probably got too concerned about the charging. Hopefully, including a very small LED voltmeter in the battery (with a push to display switch) should help keep things sweet.

B
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Old 13th Aug 2020, 5:04 pm   #20
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Default Re: "Battery Manager" for a Li ion Battery

There's one beauty to lithiums, they tend to run pretty much at full capacity then fall off a cliff when drained, unlike other battery types.
For charging, it needs to be balanced.
Depending on what the OP plans to do, wouldn't it be a better idea to source a flat type lithium pack, as found in model aircraft?
Chargers for these packs are readily available, plug and charge!
Just a thought anyhows.
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