What Have You Done To Your Tdm Today?

[fixitsan]

Popped it on the lithium battery charger, dusted it off lovingly and put it back under it's wool blanket until better weather.

 

Lee, which model of battery do you have ?

 

Some of the lithium batteries, mine included, have a built in charge/optimiser circuit, to prevent overcharge/overdischarge and it also 'balances' the cells to keep them at equal voltage, which allows a standard charger to be used.

 

About 2/3rds down this page there is a photo and description of what the management/balancing circuit is for, and what it looks like in a lithium motorcycle battery

http://www.fastbikegear.co.nz/index.php?...&chapter=1

 

Any battery with one of those circuits will allow the use of a standard 12V charger. An old style mains transformer and rectifier type charger is not ideal (the rectified AC peaks at 50Hz can confuse the internal circuit), so a modern 'smooth DC' linear or switch mode supply is best.

 

A simple way to tell if your battery has a management circuit is to connect a small load to it and let it discharge while monitoring the voltage. With mine, branded as 'Shido' once the voltage drops to about 10.5V the internal circuit disconnects the cells from the terminals. At that time the battery presents as completely flat. This is something which I think will cause some people to throw them away early, and in fact that's how I bought mine for £10, taking a punt on the basis that it had just reached the 'low voltage cutoff limit'. I applied 14V DC from a bench power supply and after 2 minutes the management circuit kicked in, reconnected the cells and I measured 11.2V (off charge)....then proceeded to charge as normal.

 

The benefit of a managed battery is that unlike a lead-acid, where a vehicle alarm or another parasitic load constantly slowly flattens the battery, ruining the cells, the management circuit protects the cells from overdischarge (which makes lithium 'sensitive')  and it means that the chance of recovering a lithium which has drained down is higher than with a lead acid. It does also mean though that when the volts drop to 10.5 the alarm, any tracker or other permanently connected parasitic device suffers a power cut.

 

A bit on the balancing circuits, there are two types, known as passive and active.. A passive circuit is one which acts as a voltage limiter on each of the internal 4 cells (3.6V max each for LiFePO4 ). As each cell reaches peak voltage most of the charge current is bypassed around that particular cell through a resistor, until all 4 cells are at peak voltage. An active management circuit is one where during the charge cycle, power for one of the cells which is charging more quickly is shuttled to the cells which are charging more slowly, taking away the need to allow time after charging for balancing to occur

 

I think most of the lithium vehicle batteries use passive balancing, and this means that the balancing action of the circuit doesn't work unless the input voltage is at least equal to the battery's maximum voltage. It also means that if you monitor the charging current, even when the cells are fully charged you might measure some current flow, because the charge current is then being bypassed through the internal resistors ....assuming the management circuit doesn't disconnect at that point, because some of them don't disconnect at full charge, yet.

 

Most of this I had to learn for building the battery management circuit of my electric bike, with 800 individual cells !  (Arranged into groups of 18 series 'packs', based on my lead acid management system) https://www.youtube.com/watch?v=tlnbSwkpMcc

 

Bottom line, if the battery has a management circuit then a nice smooth 14V DC supply can charge it. When you think about it, when you fit a lithium battery you don't need to make any modification to the bikes charging circuit, it just supplies a reasonably smooth 13.8 - 14.2V....so if the bikes charging circuit which used to charge the lead acid can work, so too can a bench supply, or modern switch-mode charger.

This post was last modified: 20-02-2021, 11:04 AM by fixitsan.