Knowledgebase

[test edit by test hamster] 

BMS : Battery Management System ( active ).. and in other cases... Battery Monitoring System ( passive )

Why do you need a BMS ?

I'm not going to go into too much detail but here are the things you need to know..

A battery pack is made from a group of cells, in a perfect world these cells would all charge and discharge equally, in harmony and the BMS would pretty much not be needed or just sit there and do nothing fancy....

Lithium batteries have a voltage range they must be kept within ( varies by chemistry ) , over-charge or over-discharge them and they die a quick death, to prevent you from doing that, the BMS will shut you down if you do something bad or if there is a problem with one or more cells in the pack alerting you to a problem before you do yet more damage.

Alot of people, myself included, use lithium packs without a BMS, and by never, i repeat.. ' NEVER ' draining the pack to full capacity and by charging below maximum high voltage this can work... but it's not for everyone..

Not only can a BMS actively cut off the power, it can also " Ballance " the cells while charging, this simply means making sure that no single cell ( or parallel group of cells ) is allowed to go over-voltage and will drain energy from the higher voltage cell(s) to allow the lower voltage cells to catch up.. This, in a way, insures maximum capacity from the pack and prevents cells from drifting apart over time and cycles..

Some BMS's only monitor pack voltages and report to a secondary system, some of them tie into the throttle wires or activate the controller's E-brake signal to prevent you from using any more energy keeping the empty cells safe..

And, sometimes a faulty BMS can itself cause a healthy pack to quit working and even kill cells .. so it's not a perfect world..

a BMS needs to be sized to the battery pack's limits, and to the system it will operate, some BMS's do " Over-Current Protection " acting like a breaker, they can be reset by disconecting the pack and reconnecting it while some others will auto-reset once the condition that caused them to " trip " is corrected....

Each battery type has specific charging needs, in general, you should not use a charger meant for one type for other chemistry types... there are exceptions but educate yourself before attempting such things..

SLA : A 12v lead acid battery is made up of 6 x 2v cells in series inside one block, it should be charged to 14.5v and then trickle charged to keep it into the 13.x volts..

Nicad/Nimh : each cell is 1.2v but when charging they can peak well into the 1.7v range and a good Nickel charger should have a temperature probe inside the pack to detect heat increase at the end of charge.. once full charge has been detected a nickel charger can ballance cells in series by trickle charging the pack at low rate for a period of time, Ni cells can tolerate low rate over-charging

Lithium : varies by chemistry but they all should have a CC~CV ( Constant Current, then Constant Voltage ) charge profile.