# Designing (or buying) an undercurrent monitoring device for a 48V lithium (LiFePO4) battery charger

#### manincharge

Joined Dec 25, 2018
1
I recently purchased a 48V 200AH Lithium Iron Phosphate (LiFePO4) battery from China for a tiny-home application. It is composed of 16 3.2V cells, for a nominal voltage of 51.2:

https://www.aliexpress.com/item/Ele...ication-base-station-back-up/32860113530.html

The battery does not come with an AC charger, and I’ve been trying to rack my brain around how exactly I will accomplish this task.

The instruction manual for the battery states that the battery is to be charged at at a constant current (66A) until 3.65V x 16 (58.4V), and then to continue charging at a constant voltage of 58.4V (as the current drops off). Charging is to be terminated when the current tapers to 6.6A.

I’ve searched for a charger that can accomplish this, and so far have fallen flat. The closest that I came was this product:

https://www.aliexpress.com/item/LTO...harger-54-6V-24v-36V-48V-60V/32904435102.html

The charger can be set to constant current or constant voltage charing modes. However, it does not have stopping criteria. I was thinking of just setting the device on constant voltage mode. (The initial current would be limited by the input power, and thus would be under the 66A.) The question is - is there something that I can build or but that would cut off the battery charging when the charging current drops to below 6.6A.

Does anyone have any ideas? Am I thinking about this right, or dose anyone else have any advice? I don’t have a ton of experience with electrical circuits, but can learn quickly!

#### Hymie

Joined Mar 30, 2018
964
If you can charge the battery pack over a longer time period, a lower powered charger will be much less expensive that the item you linked.

Many ebay sellers sell Battery Monitoring/Management Systems (BMS) as shown in the link below.

In terms of the battery pack protection, (besides controlling the charge current) the circuit is monitoring the voltage across each cell – otherwise there is no way to guarantee that one of the cell voltages in the stack is exceeding that allowable (normally 4.2V).

But you would need a PSU with an output of 60Vdc at an appropriate current to use with the item below.

#### oz93666

Joined Sep 7, 2010
737
There is much confusion and misunderstanding about charging lithium cells , mainly caused by manufacturers ...

Most small domestic battery chargers do charge at constant current , and near the end of charge reduce current ... And this is what your manufacturer has recommended for your big unit ....

Lithium cells do not need to be charged at constant current , these are very often used for solar applications , where charging current is constantly changing throughout the day , and if clouds pass over...

What your manufacturer should have said is "charge at no more than 66A untill 58.4V is reached" ... the lower the charge current the less stress for the cells , and longer life for the pack .... but you might be in a hurry , so up to 66A is tolerable ..... you can go over 66A but this will impair life considerably ...

As for reducing current near end of charge , again this is not essential ,manufactures only recomend it because it will get the absolute maximum storage without impacting life..... much better to stop charging before max recommended voltage is reached , this will extend life of battery massively... but reduce stored energy ...

So many chargers will suit , keep the current as low as possible ... that's if you are not in a hurry ... but don't go over 66A

A 10 A charger will still fully charge in 20 Hrs ... or a 20A in 10 Hrs ... the charger will be cheaper and the battery will last longer ... just get the charger to switch off at 58.4V , then over the next 12Hrs the voltage of the pack will drop very slightly ... no problem , by this method you will still get about 98% full capacity , and a slightly longer pack life .

A $5 device like this .... will switch on the charger when voltage of the pack drops below a level you set ... and will switch off at the level you set You can feed it with any supply (60v and over) 1A to 30A depending on how quick you need to charge ... good settings would be on at 55V off at 58V ... but if you can set them lower and still get the storage you need , you can double or tipple the cycle life of your pack. ... total cost for supply and control about$20-40 depending on current.