# Battery Charging Using Battery University Information:

How would I use this information from Battery University to properly charge a car starter/ing battery--with the following specs: Vnom=12, CCA=640, CA=800, RC=110 minutes, AH=?

I plan to use a regulated bench power supply for charging. The way I see it, I should setup the max charging A & V controls, ensure the electrolyte levels are correct, connect the battery to the power supply & after the battery has reached full charge (i.e. when the charge current has dropped to 3 to 5% of the AH rating of the battery), I would then change the max charging V to the float V (maintenance charge V) or disconnect the battery, rest it (0h-24h), test it, & put it into service if it is good. Is that correct or did I miss something in my understanding?

What I don't know for certain is:
1. The AH rating of the battery (i.e. how this is calculated properly)
2. The max A for CC charging
3. The max V for top (saturation) charging
4. What exactly does 3% to 5% of the AH rating mean? Does this mean when the charge rate in amps drops to 3% to 5% of the battery AH rating (e.g. charger A = batt AH * .03 or 05)?
5. What exactly is the correct float V?

1. RC is how many minutes the battery will last at 25A discharge. Divide by 60 to get discharge time in hours, then multiply by 25 to get the effective capacity. Taking into account that capacity is reduced at high discharge rates, the nominal capacity of an 'RC110' battery is about 60Ah.

2. Charge current should be kept below 0.2C ('5 hour' rate) but 0.1C is better for the battery. At higher current the battery reaches peak voltage sooner but still takes the same time to top off, so there isn't much advantage in using a higher rate. 60Ah x 0.1C = 6A.

3. You want to go as high as you can without gassing, but the critical voltage is temperature dependent. If you can't measure the internal battery temperature accurately and provide the correct compensation then charge at room temperature and stay below 2.35V/cell (14V).

4. 3% to 5% of the AH rating means just what it says. However 3% of 60Ah is 1.8A, which is quite high. A good battery should go much lower than this provided that it isn't gassing. The best way to ensure that the battery gets a full charge is to wait until the current stops dropping.

The 'topping' charge usually takes considerably longer than the initial 'bulk' charge, and must not be cut short if you want the battery to last.

1. The 'correct' float voltage is 2.25-2.27V/cell (13.5-13.6V) at room temperature, but anywhere between 2.2V and 2.3V (13.2-13.8V) should be OK.
• Very good response! Just a few questions... 1. 110/60*25=45.83Ah (not 60Ah). 60/45.83=~31% higher. Is there some unwritten rule that says to get nominal capacity one should increase the 45.83 Ah by ~31%? 2. I agree that .1C or .2C are acceptable bulk charge rates, however, is there any downside to using a max of 2A for the max bulk charge rate (other than longer charge time)? 4. At what A reading is low enough to consider the batt fully charged? 100mA? 50mA? 0mA? 5. If the temp fluctuates above/below room temp when & how does that affect the recommended float charge V? – DIYser Feb 2 '16 at 23:38
• 1. It's just an estimate. Capacity is measured at 0.05C (20 hr rate). At 1C you typically only get ~50% of that. – Bruce Abbott Feb 3 '16 at 0:24
• 2. 2A is fine, just takes longer. I charge my 60Ah battery at 3A, 0.3A cutoff, 2.35V/cell. – Bruce Abbott Feb 3 '16 at 0:39
• 5. The article says voltage should be adjusted by -3mV/°C ie. -30mV at 35°C. – Bruce Abbott Feb 3 '16 at 0:39