Resistance is Futile.

Lotta google hits on this. Love your title.

We call that CEF

Charge Efficiency Factor

As a rule, 88 - 92% for the FULL, TOTAL, and COMPLETE charge cycle. Not just measured during a part of the cycle.

Battery University:

The resistance of lead acid goes up with discharge. This change is caused by the decrease of the specific gravity, a depletion of the electrolyte as it becomes more watery. The resistance increase is almost linear with the decrease of the specific gravity. A rest of a few hours will partially restore the battery as the sulphate ions can replenish themselves. The resistance change between full charge and discharge is about 40%. Cold temperature increases the internal resistance on all batteries and adds about 50% between +30°C and -18°C to lead acid batteries. Figure 6 reveals the increase of the internal resistance of a gelled lead acid battery used for wheelchairs.




Figure 6: Typical internal resistance readings of a lead acid wheelchair battery. The battery was discharged from full charge to 10.50V. The readings were taken at open circuit voltage (OCV).
Cadex battery laboratories.

So you can "force" it with higher volts or higher amperage. But either way, the lower your voltage is Open Circuit, when you first start charging at the battery, the higher the internal resistance of the battery, and the more heat the battery acid is going to develop and generate.

If I can program a PWM charger to get me 15.0V when in bulk mode charging, will it shorten the amount of sunlight charging time I need, say, on a partially cloudy day, to top the batteries off? Still trying to figure out what this portable folding 120w solar panel might make on cloudy days, but i missed my chance yesterday, we don't get too many cloudy days here in So Cal.

MEXICOWANDERER wrote:
We call that CEF

Charge Efficiency Factor

As a rule, 88 - 92% for the FULL, TOTAL, and COMPLETE charge cycle. Not just measured during a part of the cycle.

So 8 to 12% lost at the battery, and another 10% lost at the charge controller if it's PWM. Lotta juice and energy just going "poof!" and being lost in space.

I've been measuring battery resistance for years. It's not that difficult. You'll need an amp meter and voltmeter.

1. Measure charging current into the battery (I_1) and voltage at the battery (V_1).
2. Turn on a load that's about I_1/2. (Half of I_1)
3. Immediately measure battery current (I_2) and voltage (V_2).
4. Calculate battery resistance: R_bat = V_1 - V_2 / I_1 - I_2

Run the test a few times to get consistent results. You can also do the test going from 1/2 current to full current. Results should be the same.

Resistance is dependent on SOC. It increases above 75% SOC.

A LA battery could have about 12 mohm resistance. AGM have about 1/2 that amount.

Two batteries in parallel will give you half the resistance of one battery.

Sal

Takes energy for electrolysis, and swapping electrons. Then there's heat. Measuring battery resistance is sort of tricky - valid for some things, wildly inaccurate for calculations. Any chemical reaction is not linear. Damned tricky to express efficiency in a simple formula.

The most EFFICIENT charge regimen energy wise relies almost totally on charge rejection. The least efficient charging comes during the last 20% of the recharging cycle. The latticed sulfation department laughs and says it doesn't care what the charge efficiency department thinks.

I've had repeatable results measuring battery resistance by:

R = delta V / delta I

From Battery University:
"DC load measurements work well to check large stationary batteries, and the ohmic readings are very accurate and repeatable."

Sal

MEXICOWANDERER wrote:
Takes energy for electrolysis, and swapping electrons. Then there's heat. Measuring battery resistance is sort of tricky - valid for some things, wildly inaccurate for calculations. Any chemical reaction is not linear. Damned tricky to express efficiency in a simple formula.

The battery makes a perfect Thevenin model.

So, measure the open circuit resting voltage (Voc).

Connect a known resistive load (Rl) across the battery terminals.

Measure the loaded battery voltage across the load (Vl).

The internal resistance of the battery (Rb) = Rl*(Voc/Vl - 1)

Oops, I reread your post and you would need to test the SG and then measure for Rb at various SG and then plot.

With that said, the resistance of the battery increases over time and the curve will change shape slightly too, so this might really be futile...

You would need to do it perhaps ten thousand times during a charge cycle to attain a smooth enough profile for a graph. If resistance was easy to factor then charging wattage would be the same throughout the cycle.

Energy In

Versus Energy Out

Using a recharge amperage regimen of say 5% of amp hour rating versus 40% of amp hour rating will provide a meaningful differential.

Spot resistance checks are useful up to a point.