# Overcurrent protection solution with low resistance

#### lifebatt

Joined Oct 6, 2009
3
I want to hold a group of cells at constant voltage (anywhere from 3.0V to 3.6V). I plan to hold them in parallel so I can use a single power supply. Once the cells reach stabilization at their designated voltage the current should reduce to under 1mA. The problem is if a cell shorts out it will slowly short out the other cells too! I need a reliable solution to drop one cell from the circuit if it shorts out (soft or hard short).

So, I want something to detect an anomaly in the current being requested by each cell that will break it off. I'd like to hear your ideas; any help is greatly appreciated! More details if needed.

#### lifebatt

Joined Oct 6, 2009
3

#### MikeML

Joined Oct 2, 2009
5,444
Use a PolyFuse in series with each battery, then parallel the fuse/battery strings. If a battery is shorted, the PolyFuse will open, allowing the remaining batteries to charge normally.

#### lifebatt

Joined Oct 6, 2009
3
Where can I find this Polyfuse? What sort of resistance will they be adding?

#### MikeML

Joined Oct 2, 2009
5,444

#### russ_hensel

Joined Jan 11, 2009
825
1 Why should a cell short out?
2 A diode in series with each cell will stop reverse current with a cost of .1's of a volt on the cell. For a si diode about .7 v which is a lot for a single solar cell.
3 The current to "blow" a poly fuse may not be available.
4 A skematic of you setup and data for the cell would be useful.

#### bountyhunter

Joined Sep 7, 2009
2,512
I want to hold a group of cells at constant voltage (anywhere from 3.0V to 3.6V). I plan to hold them in parallel so I can use a single power supply. Once the cells reach stabilization at their designated voltage the current should reduce to under 1mA. The problem is if a cell shorts out it will slowly short out the other cells too! I need a reliable solution to drop one cell from the circuit if it shorts out (soft or hard short).

So, I want something to detect an anomaly in the current being requested by each cell that will break it off. I'd like to hear your ideas; any help is greatly appreciated! More details if needed.
All the Sony battery packs contain a small PCB with a sense/cotrol IC's that control mosfets to open the current path to their LI-ION batteries. You can use a similar approach. In the national Semi chip we designed for this purpose, we sensed the curent through the lead frame of the IC (voltage drop) to trigger a detector to open the FETs.