Odd 12V Lead Acid Battery phenomenon - FYI

Beau Schwabe

Regardless of the battery chemistry, this is more of an FYI to express the importance of a Charge balancer circuit (BMS) when you have multiple batteries in series driving a respectable load.


While looking at the old SLA batteries to the 4-Wheel scooter ... http://forums.parallax.com/showthread.php?p=869701

Two of the batteries were well on their way towards cell reversal measuring about 6V a piece when they are supposed to measure 12V.

The oddball battery measured 24V when it was supposed to only be 12V and shows a respectable amount of current. It actually appears to be healthy, compared to the others.


So, the original charger would "see" 36V and think that the batteries were adequately charged (24V + 6V + 6V = 36V) only to have a very short running life while powering the scooter.

A simple shunt regulator circuit on each battery pack tuned for 14.4V would have probably saved the batteries from being over charged, and given the undercharged batteries a better chance to recuperate from being deep cycled from the scooter.

Something like the circuit here ... (See Figure 21) ... focus.ti.com/lit/ds/symlink/tl431.pdf


Edit:

Turns out the 24V is bad, and the others are barely hanging on. Under a load for about an hour, the 24V quickly has come down to almost nothing. I'm a bit puzzled how the voltage was able to initially maintain that high myself, but still. I believe that a shunt regulator on each 12V battery would have prevented this.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 1/4/2010 7:25:12 PM GMT

Phil Pilgrim (PhiPi)

Beau,

The 6V batteries I can understand. But 24V? What transformation could have occurred to coerce a lead/lead dioxide cell to produce 4V instead of 2V? (Are you sure there aren't twelve cells tucked away in that one? )

-Phil

skylight

were the voltages read under load using an appropriate meter?

Beau Schwabe

skylight,

I'm certain the meter wasn't the problem, however a weak 9V battery within the meter could have been a contributor. The meter itself is a Protek D981 and the load that I placed across the battery for approximately 1 hour was a 24V 120Watt DC motor (Model #MY6812)

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

BradC

Chains of lead acids are quite fussy to properly maintain. We have 2 chains of 4x6V on the boat, and they can be quite difficult to keep balanced. This is of course made somewhat worse by having a mix of batteries from different manufacturers and ages (we only replace when we have to as @ $180 each it's quite an investment to do the whole lot at once).

We've finally (after 3 boats and 28 years) nailed a process that works *really* well, and we seem to get in excess of 8 years life from a battery now (much to the total disbelief of the guy who sells the batteries). You can't use this process with sealed cells as the balancing relies on boiling off some electrolyte but for flooded cells it really works a treat (in lieu of using shunt regulators which would help balance the batteries, but not the individual cells).

We charge the pack up to nominally 28.8v and hold it there for about 4 hours. Then, once a month, we increase the charge voltage to about 29.8v for an hour. The extra voltage causes batteries that are already fully charged to slowly boil, but the current they pass while boiling helps bring the less charged cells up to par. The batteries are kept floated at 27.6v either by solar or a mains charger. The charging is done by constant current up to 27.6v (20A), then constant voltage to 28.8. The balance charge is constant current to 29.8v (5A) but never for more than an hour, and never if the ambient is above about 24C (given they are sitting in a boat hull they rarely see more than an ambient of 22 anyway).

Interestingly, we've found that after a couple of years it's not unusual for the cells in a battery to go wildly out of balance, causing all sorts of issues. This balancing process helps keep each individual battery balanced also which has proven to extend their life considerably.

It helps to have a condensing clothes drier which gives us an abundant supply of distilled water, although you actually use very little to keep them properly topped up. The balance process boils of a lot less electrolyte than you might think, but it's a good idea to keep the area well ventilated.

When we bought this boat we were destroying a set of batteries in about 8 months, then we realised the idiots who built it derived the 12V supply by tapping half the 24V banks. You can imagine what that does to the balance of a bank!

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Life may be "too short", but it's the longest thing we ever do.