Light Weight battery
Sep 22, 2016, 08:10 PM
#20
EvoM Guru
iTrader: (1)
Originally Posted by heel2toe
Doesn't matter. Once the car is running the alternator takes over.
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jedibow (Sep 24, 2016)
Apr 2, 2020, 03:02 PM
#26
Bumping this one instead of a new one. Ok Lithium Batteries - don't jump it, don't drain it, blah blah blah. But what gives when it comes to charging/storage of the thing. Half of what I read says even the expensive Li chargers can't be left on because it'll wear out the battery. Driving the car with the Alternator is basically the same thing. However for us race cars that sit for MONTHS at a time - whats the strategy? Kill switch is easy but me thinks why don't these chargers "shut off" until it detects a voltage drop? Laptops and such are LiN whereas I believe these are LifeP04. #confused.
In further efforts to loose weight I swapped from Group30 AGM to a Group30 Li battery and saved roughly 16lbs. But I don't want to burn this thing up.
In further efforts to loose weight I swapped from Group30 AGM to a Group30 Li battery and saved roughly 16lbs. But I don't want to burn this thing up.
Apr 2, 2020, 06:21 PM
#27
EvoM Community Team Leader
It all comes down to the BMS that the battery is using. LiFePo4 cells are fully charged at 3.6v per cell and a battery pack for a car will have a 4S configuration, or 4 cells in series for a fully charged voltage of 14.4 volts. LiFePo4 cells will be damaged if they are charged below freezing. So as long as the BMS keeps the cells balanced and has a low temp thermal cut off it should prevent you from damaging the cells from leaving it on a charger. As long as your charger doesn't exceed 14.4 volts. Any decent BMS will have under voltage protection to prevent damage to the cells from over discharge, but since you probably don't have the specs of the BMS itself you should follow the recommendations of the battery manufacturer.
Through my own testing with raw cells in my own packs I can tell you they don't like extreme temps in either direction. I would routinely lose the cells closest to the engine with the pack under the hood daily driving, so I would suggest you relocate the pack to keep it as far away from heat as possible. LiFePo4 have a non destructive fail mode compared to Lithium-ion, so there is no danger of them blowing up or catching fire if they do go bad, but they can crank out the amps so an electrical short can quickly become a fire if not properly fused / protected.
Through my own testing with raw cells in my own packs I can tell you they don't like extreme temps in either direction. I would routinely lose the cells closest to the engine with the pack under the hood daily driving, so I would suggest you relocate the pack to keep it as far away from heat as possible. LiFePo4 have a non destructive fail mode compared to Lithium-ion, so there is no danger of them blowing up or catching fire if they do go bad, but they can crank out the amps so an electrical short can quickly become a fire if not properly fused / protected.
Apr 3, 2020, 03:16 AM
#28
Bumping this one instead of a new one. Ok Lithium Batteries - don't jump it, don't drain it, blah blah blah. But what gives when it comes to charging/storage of the thing. Half of what I read says even the expensive Li chargers can't be left on because it'll wear out the battery. Driving the car with the Alternator is basically the same thing. However for us race cars that sit for MONTHS at a time - whats the strategy? Kill switch is easy but me thinks why don't these chargers "shut off" until it detects a voltage drop? Laptops and such are LiN whereas I believe these are LifeP04. #confused.
In further efforts to loose weight I swapped from Group30 AGM to a Group30 Li battery and saved roughly 16lbs. But I don't want to burn this thing up.
In further efforts to loose weight I swapped from Group30 AGM to a Group30 Li battery and saved roughly 16lbs. But I don't want to burn this thing up.
George
Apr 3, 2020, 07:41 AM
#29
Ya I got a 2 month old Braille G30 (not a fan originally because they're resellers not manufactures) off a Pcar owner at the shop for 100 bucks because he parted the car. 5.5 lbs it's sick light. The Braille 6 amp charger on their site states it charges to 14.4 volts and then lights up when it's done - so if the battery "lives" at 14.4 why is one piece of paper saying don't leave it unattended, don't leave it on, leaving it on will wear it out as if driving it 24/7/365, etc etc. While the other says leave it on if it sits for more then 2 weeks.
Apr 3, 2020, 07:43 AM
#30
Biggiesacks nailed it. Balancing the cells is key.
Ideally, all four of the cells in series get charged equally: 3.6 + 3.6 + 3.6 + 3.6 = 14.4V, and all is good.
In the real world, cells aren't perfectly matched, and they age differently. Over time you could end up with something like 3.3 + 3.5 + 3.7 + 3.9 = 14.4V. The whole pack will still read 14.4V and accepts a 14.4V charging voltage, but one of those cells is taking more stress and damage than the others. It works for a while, but now the cells are charging differently and some may have developed self-discharge problems.
Eventually, the cells might become so imbalanced that one cell is pushed out of the safe zone. If you get something like 3.6 + 3.6 + 4.4 + 2.8 = 14.4V, that 4.4V cell is going to be damaged. Best case, the pack dies quietly. Worst case, it catches fire. One of our local autocrossers had a DIY lithium battery pack catch fire under his hood for this exact reason.
The problem is that the 2-wire chargers can only see the series voltage across the pack, not the individual voltage of the cells. If the pack becomes unbalanced, charging it to the set point might still overcharge a single cell in the series.
Laptop batteries and other systems have additional wires in between each cell. This allows the pack to burn off some energy from the overcharged cells, keeping the pack in balance.
A balancing charger for a car battery would have at least 5 wires connecting to the battery pack: 2 for +14.4V and GND, and another 3 for the connections between the cells (3.6V, 7.2V, 10.8V). The really good packs and chargers also have temperature sensors to monitor for thermal problems, but I don't know if anyone is doing that in the automotive space.
The alternative is to have a battery management system (BMS) in the battery itself to keep the cells balanced. This is more common in automotive batteries because it doesn't require a fancy charger, but to be honest I don't know which vendors actually do it. The idea here is that each cell in the pack has a circuit that limits it to a maximum voltage (3.6V). If the cell is charged higher than that voltage, it burns the excess energy off as heat. It's not really ideal because high charging currents could become a lot of heat, but it generally works for low amperage trickle charge. This is what I'd look for in a basic lithium ion battery pack.
The best option I've seen so far (not an exhaustive search) is Antigravity's RE-START series: https://antigravitybatteries.com/restart-technology/ They have an automatic disconnect if the battery is starting to get too discharged. If it trips, you press the button to reconnect the battery and ideally have enough power left to start the car once more. They also have built-in cell balancing.
Ideally, all four of the cells in series get charged equally: 3.6 + 3.6 + 3.6 + 3.6 = 14.4V, and all is good.
In the real world, cells aren't perfectly matched, and they age differently. Over time you could end up with something like 3.3 + 3.5 + 3.7 + 3.9 = 14.4V. The whole pack will still read 14.4V and accepts a 14.4V charging voltage, but one of those cells is taking more stress and damage than the others. It works for a while, but now the cells are charging differently and some may have developed self-discharge problems.
Eventually, the cells might become so imbalanced that one cell is pushed out of the safe zone. If you get something like 3.6 + 3.6 + 4.4 + 2.8 = 14.4V, that 4.4V cell is going to be damaged. Best case, the pack dies quietly. Worst case, it catches fire. One of our local autocrossers had a DIY lithium battery pack catch fire under his hood for this exact reason.
Kill switch is easy but me thinks why don't these chargers "shut off" until it detects a voltage drop? Laptops and such are LiN whereas I believe these are LifeP04.
Laptop batteries and other systems have additional wires in between each cell. This allows the pack to burn off some energy from the overcharged cells, keeping the pack in balance.
A balancing charger for a car battery would have at least 5 wires connecting to the battery pack: 2 for +14.4V and GND, and another 3 for the connections between the cells (3.6V, 7.2V, 10.8V). The really good packs and chargers also have temperature sensors to monitor for thermal problems, but I don't know if anyone is doing that in the automotive space.
The alternative is to have a battery management system (BMS) in the battery itself to keep the cells balanced. This is more common in automotive batteries because it doesn't require a fancy charger, but to be honest I don't know which vendors actually do it. The idea here is that each cell in the pack has a circuit that limits it to a maximum voltage (3.6V). If the cell is charged higher than that voltage, it burns the excess energy off as heat. It's not really ideal because high charging currents could become a lot of heat, but it generally works for low amperage trickle charge. This is what I'd look for in a basic lithium ion battery pack.
The best option I've seen so far (not an exhaustive search) is Antigravity's RE-START series: https://antigravitybatteries.com/restart-technology/ They have an automatic disconnect if the battery is starting to get too discharged. If it trips, you press the button to reconnect the battery and ideally have enough power left to start the car once more. They also have built-in cell balancing.