# What voltage do you measure while implementing the charge algorithm?

So the charge algorithm for recharging a battery is (I will be using a lithium-polymer):

1. Pre-charge
2. Constant current charge
3. Constant Voltage charge

But I have learned that the open circuit voltage of battery is different to the voltage measured while charging. So my question is what/which voltage do u measure while you charge the battery. How do u measure this. I have searched for this, before asking the question here. So if anyone could direct me in the right direction would really appreciate it.

The state transition from pre-charge to constant current (fast) charge takes place when the battery voltage first reaches a preset voltage with the pre-charge current flowing.

The state transition from fast charge to constant voltage takes place at when the battery voltage first reaches the termination voltage, with the fast charge current flowing.

The constant voltage charge terminates when the charge current drops to the taper current threshold. Obviously, the termination voltage must be applied for the current to flow.

(Of course there are usually time, temperature and other ways to terminate early if the battery is acting unhealthy)

You might find looking at the data sheets for charging chips useful, but there is no substitute for the detailed recommendations from your battery manufacturer. If you can't get that, try to find a similar battery from a reputable manufacturer.

• thank you for your reply. I understand that, but if I was to make a charger myself. What voltage and current do I keep track of (open circuit or voltage under-charge?) That is my question. That measurement do i keep track of so that i program my controller to make all those state transition you talk about. What should I be measuring for me to program my controller – user36043 Jan 24 '14 at 8:56
• You have two answers that clearly say "under charge" where voltage is measured. I don't know how to make it any more clear. – Spehro Pefhany Jan 24 '14 at 12:54

Measure the voltage under charge. So you don't have to stop charging in order to measure it.

Various charger ICs already do this, e.g. http://uk.farnell.com/intersil/isl9214irz/ic-sm-charger-li-ion-polymer/dp/1361033 .

• Thank you so much for your reply. I have to design my own for a specific application. I want to know specifically before pre-charging(to see is the battery is below approx 2.8v, i am assuming this is a open circuit measurement) and at the end of charge(to measure current falls below 0.1, under charge). will it influence the battery condition in the long run. Because i am assuming voltage under charge is slightly lower than open circuit voltage. Will it be fine measuring the voltage under charge, to create a stop charge condition when the current has dropped to less than 0.1C? – user36043 Jan 24 '14 at 9:59
• Voltage under charge will be slightly higher than the open circuit voltage: that's how you push charge into the battery. – pjc50 Jan 24 '14 at 10:00
• Okay...so then it is wise to measure under charge. thank You – user36043 Jan 24 '14 at 10:23