# interrupting a LiPo balancing circuit

I have an application that's powered by a typical 3S LiPo battery pack and charged through an MPPT solar charger. The charging circuit in the link doesn't balance the battery pack, however, which means that I have to do it some other way. It's trivial enough to buy an off-the-shelf balancing circuit, but I need to be able to shut it off when not charging so that it doesn't drain the battery.

The question is this: given a standard xS LiPo battery pack, and access to the CHRG pin off the charger (meaning a signal that goes high when charging), where do I need to interrupt the balancing plug to safely shut down an arbitrary external balancing circuit? Do I have to interrupt all x+1 lines, or can I just interrupt one of them?

• Leaving one only line to balancer is OK as long as it does not have any 'backdoor' path. eg \if you interupt blue red to b plug and leave black AND balancer also has red feed from XT60 plug etc then balancer now has two lines and MAY discharge battery | A well desigbned balancer should be able to be turned off completely with all wires still connected. – Russell McMahon Oct 14 '16 at 12:18
• @RussellMcMahon I'm assuming that I would be wiring up a MOSFET of some sort and re-routing one of the balancing wires through it. I'm not sure I follow your statement though - would it be enough to just interrupt the black (-) wire on the balance plug, for example, or would I need to interrupt the blue and red instead? – kolosy Oct 14 '16 at 14:17
• To be safe with an unknown sourced balancer you can connect 0 or 1 wires. ie no ciruit is possible, If you connect ANY 2 wires by any pathe then a circuit is able to draw current when notionally "off". Whether it does so is up to the designer. – Russell McMahon Oct 17 '16 at 13:11

## 2 Answers

Leaving one only line to balancer is OK as long as it does not have any 'backdoor' path. eg if you interrupt blue red to b plug and leave black lead to balancer
AND
balancer also has red feed from XT60 plug etc then balancer now has two lines and MAY discharge battery

A well designed balancer should be able to be turned off completely with all wires still connected.

Balancing requires shunting the current of fully charged cells while allowing charge to complete on the undercharged cells.

In-expensive bypass shunt types handle about <<10% cell capacity inbalance so that the 10% of the charge power can be reasonably dissipated of each cell charge power being pumped. With individual cell monitoring, to limit cell voltage to 4.2V then when charger is switched off the cell voltage decays to 3.8V or 10% below peak charge the balancer should not be loading any individual cell.

A smart balance charger uses Sendyne's novel patented method half-bridge switched inductor method, current can be more efficiently bypass the fully charged cells.

Look for detailed specs on off-the-shelf balancers, rather than guess what "some Ebay offering" is selling without specs. Ask the supplier.