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Some People Claim that Lithium Batteries don’t need a BMS (Battery Management System).
For us it’s a question of safety first. It is important to inform customers of the potential risks.
Most times it is true that Lithium batteries will stay relatively balanced for a period without any external balancing or monitoring. However there are some potential risks. Lets have a look at some "what if?" scenarios in a 12V (4 cell) battery. Remember that the upper safety limit for a LiFePO4 cell is 4.2V.
What if the battery does become unbalanced due to aging or cell IR (internal resistance) differences?
- In the worst case this would mean a pack charged to 14.6V could have three cells at 3.3V and one cell at 4.7V. This high cell would swell, heat up, vent and potentially burst with flames.
What if a car or boat alternator voltage regulator malfunctions or is maladjusted and overcharges the battery?
- An alternator can produce up to 300VDC if unregulated. If any LFP battery with no BMS checking the upper cell voltage limit was subjected to this it could fail catastrophically.
What if the battery is accidentally over-discharged?
- There can be varying degrees of this. Many times the battery can be recovered if treated properly. However if one or more cells goes to zero volts for a period or is reverse polarized by discharge currents for even a few seconds then the battery will be permanently damaged and become unbalanced. A subsequent recharge carries a significant risk of catastrophic failure.
What if one cell fails and falls to zero volts?
- This is a very rare but normal failure mode for Lithium cells. In this case the cell most often forms an internal short circuit. So if there are 4 x 3.2V cells in a 12V pack the peak charge voltage would normally be 14.4-14.6V. So 14.4 / 3 = 4.8V per remaining cell. At this voltage the remaining cells would swell, heat up, vent flammable gas, and potentially burst and catch fire.
Some rules of thumb:
- The larger the cell capacity the larger the risk of damage from a catastrophic failure.
- The more cells connected in series the greater the risk of unbalance and failure.
- Batteries comprised of series strings of cells connected in parallel present a much greater risk of catastrophic failure.
- The larger the charge and discharge currents the larger the possibility of variable internal resistances causing unbalance. Internal resistance creates heat during both charge and discharge so the highest IR cell will lose capacity first.
- If a LFP battery is over-discharged it can become unbalanced and unstable unless correctly recovered, preferably using the correct charger for the purpose.
- Using an intelligent Lithium dedicated charger with a deep discharge recovery charge curve is much safer than using a regular SLA charger.
We have a smashed safety glass window in our workshop from an exploded Winston Thundersky cell that we were testing. It proves that LiFePO4 Lithium batteries can explode and catch fire if overcharged. Or more importantly over-discharged and then recharged without checking for a dead cell. We know of at least one other vendor that has had a similar experience. Also there are many examples of battery fires where no BMS was used or where an improperly designed BMS was inactive or failed.
We do our absolute best to minimize the risk of battery failure for our customers. On larger battery packs we advocate the use of our Australian made BMS. Smaller packs are safer but also benefit from over-discharge protection.
A Battery Management System (BMS) performs two functions. The first function extends the battery life. The second works to prevent catastophic failure. No system is perfect but it is better to be annoyed by a system dropout that be alarmed by a dramatic failure.
All EV Power Pak batteries include a cell level balancing and monitoring BMS. Many other vendors do the same. We will sell you the cells. You don’t have to use BMS but it is our duty to explain the risks involved if you don’t.