Hi community, I made a 4s 4p battery pack a few months ago with used 18650 cells. Checked every cell, combined them in the best possible way respecting capacity and internal resistance. Of course added a BMS, which in my case is anctive balancer, the ones with capacitors (preferred rather than the resistance ones which balances discharging cells). Welded everything with a spot welder. The battery pack is used in a car, it is enclosed in a plastic box and works in conjunction with a capacitor pack which is the one that starts the engine. The lithium battery pack is there to cope with the high discharge rate of the caps charging them at max 20 amp. So knowing all those informations, what could be the reasons why my battery pack lasta only a fes months/max 1 year? Thanks for any advice.
Why my battery pack lasts only a few months?
- Thread starter edwink89
- Start date
BiggKidd
100 W
Sounds to me like you are pushing the cells to hard. Assuming they aren't getting heat saturated under the hood anyway.
j bjork
1 MW
How do you limit the discharge to max 20A?
docw009
10 MW
The results are saying that 20A is too much for your 4P combo. All that lore and software tools for matching IR and capacity only works for low current applications. Your cells want to be all the same band and same age. Otherwise, the 20 amps isn't always going to distribute at a nice 5A per cell.
It could be extreme as 20A on two of them and 0A on the other two, Some of the long term battery experts have posted how that happens.
So sixteen cells? Spend some money and buy new brand name cells.
It could be extreme as 20A on two of them and 0A on the other two, Some of the long term battery experts have posted how that happens.
So sixteen cells? Spend some money and buy new brand name cells.
I got one of these a couple of weeks ago to diagnose some odd charging issues on my wife's car.
Not enough facts to understand what the set up is and whether the battery it tied into the car's "12v" system, including charging. Seems like there's no charge or discharge protection, so that would be hard to manage without some type of voltage monitoring.
Some questions to provoke thought about possible failure sources:
What is the exact failure that you see? Capacity loss? Capability to deliver current decrease? Voltage sag under load? Fire? ???
What temperatures does it reach in the battery compartment?
What humidity and other environmental conditions does the pack experience insde it's compartment and box?
What was the source of the used 18650 cells?
Were they identical, matched-characteristics cells, so that they all have identical resistance, capacity, and all other properties?
How used were they?
How old were they?
How were they removed from their prior usage? (how was the source pack disassembled?)
How were they prepared for the new usage?
How were they tested prior to assembly in the new pack?
What were the test results for each cell?
Which specific cells are they?
What are their capabilities? (how much current does their spec sheet from the cell manufacturer indicate they are nominally able to supply, and how much current maximum, for how long? )
Are they being charged within their current and voltage limits?
Are they being discahrged within their current and voltage limits?
Etc.
What is the exact failure that you see? Capacity loss? Capability to deliver current decrease? Voltage sag under load? Fire? ???
What temperatures does it reach in the battery compartment?
What humidity and other environmental conditions does the pack experience insde it's compartment and box?
What was the source of the used 18650 cells?
Were they identical, matched-characteristics cells, so that they all have identical resistance, capacity, and all other properties?
How used were they?
How old were they?
How were they removed from their prior usage? (how was the source pack disassembled?)
How were they prepared for the new usage?
How were they tested prior to assembly in the new pack?
What were the test results for each cell?
Which specific cells are they?
What are their capabilities? (how much current does their spec sheet from the cell manufacturer indicate they are nominally able to supply, and how much current maximum, for how long? )
Are they being charged within their current and voltage limits?
Are they being discahrged within their current and voltage limits?
Etc.
Thank you all for your quick responses. The main problem is that I lose capacity, in about a month the analog voltmeter on the cluster shows a drop of about 2 volts (from 14 to 12v), during that month the engine starts normally and an external charge is needed maybe once in a month. My smallest commute is about 20 minutes, which is enough to restore the current used for starting the engine. The cycle repeats in the next months but it get worse meaning that the time needed to charge the battery is more. After about 1 year I am not able to rely on the lithium battery pack anymore because it is not able to cope with 2 days of capacitor drain (voltmeter showing about 8 volts); if the car is used every day the engine will start, otherwise it will not. I am using a SLA battery charger instead of a lithium battery one, so maybe the fact that it is not a CCCV algorithm can be a problem.
The battery is located upward right under the hood, but behind a "firewall", a metal sheet that divides the engine compartment and the battery, so it is not suffering from overtemperature. Also i put iside the box a few desiccant bags to absorb humidity.
I do not know exactly how the cells are used and how old are they but the only data i have about them is that they are Sanyo UR18650RX cells, rated at 1950 mAmp with an average of 1700 mAmp of capacity left, they come from Bosch tools 20v battery pack and are all identical. I removed them ripping the nickel plated strips trying to not damage them and levelled the tabs with a dremel. All the cells have been tested with a LIITOKALA Lii-500 charger/tester and after fully charged spot welded.
I really don't know how are the packs charged and discharged in the car as I am not equipped with a voltage monitoring system (apart from the analog voltage meter on the cluster).
The current is limited with a 25w resistor placed between the battery pack and the capacitors on the positive. Is there a better way to limit current?
I forgot to mention that lately I added another identical pack, and the same problem happens.
Many thanks.
The battery is located upward right under the hood, but behind a "firewall", a metal sheet that divides the engine compartment and the battery, so it is not suffering from overtemperature. Also i put iside the box a few desiccant bags to absorb humidity.
I do not know exactly how the cells are used and how old are they but the only data i have about them is that they are Sanyo UR18650RX cells, rated at 1950 mAmp with an average of 1700 mAmp of capacity left, they come from Bosch tools 20v battery pack and are all identical. I removed them ripping the nickel plated strips trying to not damage them and levelled the tabs with a dremel. All the cells have been tested with a LIITOKALA Lii-500 charger/tester and after fully charged spot welded.
I really don't know how are the packs charged and discharged in the car as I am not equipped with a voltage monitoring system (apart from the analog voltage meter on the cluster).
The current is limited with a 25w resistor placed between the battery pack and the capacitors on the positive. Is there a better way to limit current?
I forgot to mention that lately I added another identical pack, and the same problem happens.
Many thanks.
rockterrabloodyfirma
1 µW
You got what I want can you send mail or phone no. To follow your path to a comprehensive batter build.
Why do you not use existing lifepo4 starter batteries?
I have an A123 40Ah Lifepo ( bought cheap from Porsche Taycan warranty claims) installed in my car and it just works fine since 4 years. But that is only a good solution if you do not use your car in freezing temperatures.
I have an A123 40Ah Lifepo ( bought cheap from Porsche Taycan warranty claims) installed in my car and it just works fine since 4 years. But that is only a good solution if you do not use your car in freezing temperatures.
Can you clarify exactly how the batteries are recharged under normal use? Is it charged from the car’s charging system (via the alternator) or by using a plug in charger? The cars system tops out at 14.5v, so would barely charge the battery lithium battery half way, so I’m guessing you have some other method?
I made this because my SLA battery died and I wanted maximum weight savings, the actual setup weights 4,1 kg. Lifepo4 starter battery is a good option, here the temperatures are moderate, but the problem is cost.
Batteries are normally charged from car's alternator but after a few weeks there is not enough charge left, so I had to recharge them with an external charger and i kept going for about a month. That is also a problem i thinked about, the lithium battery packs will always be on the low charge side, so a better option is to switch to 3,2v lithium cells.
Batteries are normally charged from car's alternator but after a few weeks there is not enough charge left, so I had to recharge them with an external charger and i kept going for about a month. That is also a problem i thinked about, the lithium battery packs will always be on the low charge side, so a better option is to switch to 3,2v lithium cells.
The A123 (now lithium works) 40Ah lifepo weights less than 7kg, including a complete functional BMS.
Before I had 12V 18Ah 6kg high discharge AGM Batteries installed, but they lasted only 3 years and in the second year I had problems starting the car after it was 4 weeks not used.
I did this also for weight saving reasons in my lightweight sportscar
Before I had 12V 18Ah 6kg high discharge AGM Batteries installed, but they lasted only 3 years and in the second year I had problems starting the car after it was 4 weeks not used.
I did this also for weight saving reasons in my lightweight sportscar
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So instead of charging to 16.8v, you’re only charging to 14.5v (both by using the cars charging system or sla charger). You could try adding a boost converter to charge the batteries to 16.5v or so and get a voltmeter like i linked earlier to monitor what’s going on. And add a real BMS to add some protection from over discharging them so much. Or just stop messing around and get the LiFEPO4 battery.
The 123A battery Dominik suggested is an excellent solution but still expensive. My second best option is to buy some new lifepo4 battery cells and make a decent pack.
What would in your opinion be a real BMS? Can you post a link or something please? My actual BMS do offer protection from overcharge and overdischarge.
What would in your opinion be a real BMS? Can you post a link or something please? My actual BMS do offer protection from overcharge and overdischarge.
scianiac
10 kW
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- Oct 24, 2018
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I would go with high power LFP (probably some cells from batteryhookup) with a BMS that has low temp charge protection, keep the supercap and wire the starter solenoid to bypass the BMS that way you don't need a several hundred amp BMS but the charging and discharging of everything else goes through the BMS. The only issue with this setup if you use the car in cold weather and the battery doesn't get warm enough to charge it will never charge, may not be an issue if the battery is in the engine bay or the car is stored in a garage warmer garage sometimes. You could get around this with a heating pad connected either to a BMS with a heater function or just to a thermal switch connected so it only is on when the car is running. I've seen some JKBMS that have a heater function for instance.
I read your first post as using an active balancer as your BMS. If it’s an active balancing BMS, then you d need to make sure it’s one that balances at partial charge, since you never achieve anything approaching a full charge, which is when most BMSs, both passive and active balancing type, trigger balancing. Have your measured you cell voltages before you use you sla charger to see if they are balanced?
I have a JK-B2A-24S with heater function installed in my scooter.
For me they have programmed the heater function wrong. They have only one Temperature range for heating and for the battery itself. If it is to cold they first cut off the battery from charging currents and then they start heating. It is not usable in a Scooter or Motorcycle with regeneration turned on. The BMS always switches the battery off and the controller gets in trouble, because it can send the energy nowhere.
Car is always stored in a garage where the lowest temperature was about 10°C and the BMS have an overtemperature protection, so tempertures are not an issue. The BMS is balancing always balancing, at full and partial charge. Lifepo4 is for sure the way to go, but can anybody suggest a better way to limit current between BMS and capacitors?
scianiac
10 kW
- Joined
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- 621
I mean a resistor is all you need to limit current between the battery and capacitor, the question though is there enough usable energy in the capacitor to start the car? I've done the math on this before with the capacitor bank I have attached to my lead acid starter battery to extend it's life and with the bank and engine I have it should be enough but remember you don't have as much as you think. You only have from the starting voltage down to around 10v or so after which it won't crank. Because of this I think it may be better to just go with LFP cells that have enough juice to start the car without the capacitor but still install it because you already have it. At that point the capacitor bank is only helping, which is to say the battery voltage will sag but however much it sags power also comes from the cap bank and then after it's started power charges the cap bank from the battery and alternator so basically you the battery has to expend about the same about of energy just over a longer period, so less during starting but then a bit longer to charge the cap bank back up.
Using just a resistor for this, the current changes depending on the voltage difference between the source and destination.
So you'd need to size the resistor so that the largest possible difference between them limits the current to less than the max current the least-capable part was able to handle, and then live with the lower currents you'd have in every other situation.
There is enough energy stored to start the engine, even at 12v. There are 3 capacitor banks rated for 500 farad each, they're all made out of 6 2,7v caps in series for a total of 16,2v fully charged, all the caps are soldered on a balancing board and that is the pack you can find on Amazon, Aliexpress, eBay etc. So if only a resistor is needed to limit the current i'm already on the good way. The actual resistor is a 25w 0,1 ohm, which shoupd limit the current to about 10/15 amp, which is ok considering the lithium cells used.
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