Battery Question (go figure)

TreeBass

Ok, so I took some readings today, after about 12 hours off the charger.

Batt 1 - 6.18
Batt 2 - 6.13
Batt 3 - 6.11
Batt 4 - 6.09
Batt 5 - 6.12
Batt 6 - 6.08

Pack Reading - 36.8

I cleaned the terminals, added distilled water a week or two ago, so I'm pretty sure I need to replace my batteries

Now to the real questions...

Not calling you out JohnnieB, but you seem to know alot about batteries

Why do we have to wait at least six hours to check?
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What is the gassing phase? My charger runs for 10-20 minutes, and shuts off (no hum)

Am I really getting a full charge?

How do I check my charger to verify?

BGW

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scottyb

Print out this attachment on battery voltage as it relates to state of charge... that will show you that some of your batteries only hold 50% of their capacity.... 50% is considered empty, we let our batteries settle (actually 12 hours + is better than 6 hours) because it matters most what a battery can hold not what a charger can bring it up to.... as you see there is a big difference.
Looks like you are in the market for new batteries.

Attached Images

SOC-new-print-003.jpg (62.6 KB, 17 views)

ajbrown

No doubt those are very low voltages, but I am not sure I jump to the conclusion that it is the batteries yet. The line above jumps out at me.

Based on what you state, I am not even sure you are charging the pack. I am not too familiar with EZGO, so will lets others comment on testing charger.

JohnnieB

1. Due to a phenomena called "Surface Charge". Basically, while the last 10-20% of the cells maximum capacity is being absorbed during recharge, the cell voltage rises to a high level (about 2.55V), but that voltage drops quickly during discharge and is more or less meaningless for determining how much energy is actually stored in the cell. Once it is depleted, there is a nearly linear relationship between terminal voltage and energy stored. It is called a surface charge because the chemical reaction is only occurring on the surface of the plates rather than within the plates.

Flooded lead/acid cells self-discharge at about 1%/day, so 6 hours (0.25% discharge) will get rid of most of the surface charge and give you reasonable approximation of the cell's State of Charge. Waiting 12 hours will give you a closer approximation. Waiting 24 Hours will put you even closer to the linear portion of the discharge curve (90% down to 10%), but then the battery is at 99% rather than 100%. If your battery voltage readings are above the charts, wait longer. Just wait about the same length of time each time you check for consistency.

Another way of getting rid of the surface charge is drive the cart for a mile or so, wait 10-15 minutes and measure the battery voltage. Waiting a few minutes before measuring is because there is a thing called "Surface Discharge" (aka Battery Recovery). If you've ever had difficulties starting a car and had the battery get too weak to turn the engine, waited a few minutes and find that the battery magically had enough power to start the engine, you've experienced it.


2. Gassing Phase is also called Absorption Phase, or at least occurs during the Absorption Phase. During the Bulk Charge phase, the battery recovers about 80-90% of whatever was discharged from it. Chemically, the sulfur ions that had gone into the lead and lead dioxide plates during discharge recombine with the water in the electrolyte to reform the sulfuric acid that was broken down in the discharge process. The conversion from electrical energy to chemical energy in the bulk phase is in the high 90% range. Once the bulk of the sulfur ions have been returned to the electrolyte, the efficiency lowers and excess energy exists, causing the cell voltage to rise, more heat to be generated and the electrolysis of the water in the electrolyte to occur. The hydrogen and Oxygen gases bubble to the surface and give the appearance of the electrolyte boiling even through the temperature is far below the boiling point of water, much less the boiling point of a 25% sulfuric acid solution.

During the last 10-20% of the charge (Storage wise, not time wise), the batteries are decomposing water and outgassing Hydrogen and Oxygen. This is why the water need to be replenished periodically.

3. Your charger is sensing a battery voltage in the 44-46V range, or has a faulty battery voltage sensing circuit.
Typically, a short charge time means the batteries are at or near full charge when recharge was initiated, but that doesn't mean the batteries are good, or bad, only that the voltage rises to the voltage that good batteries normally rise to when they are fully charged.

4. If your charger is taking your batteries to 2.55V per cell (45.9V for a 36V battery pack), you are storing the maximum amount of chemical energy that can be stored in your batteries with the amount of crystalline lead sulphate existing in the plates. Little, if anything, can be done to restore a cell's capacity once crystalline lead sulphate has formed in or on the plates.

5. Measure the charger's output voltage while charging your batteries. It should be in the 44-46V range immediately before it shuts off automatically.

I've tossed out some fairly exact numbers, but there are so many variables in battery charging and discharging that everything is only approximate.

TreeBass

thanks guys, appreceiate the explanations. I'll verify what the pack reading is when charging next trip out.

My concern is the charger at the moment, I don't have confidence in it, and dont really want to buy new batteries only to screw them up with a faulty charger.

I'm used to marine batteries, put em on charge, and they run thru the night.

Thanks again for the advice and clarification