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Topic: strange voltage jumps in lead battery? (Read 372 times) previous topic - next topic

Dec 11, 2012, 01:10 am Last Edit: Dec 11, 2012, 01:13 am by chefkoch84 Reason: 1
Hi all,

I have a strange phenomenom here:

I am measuring the voltage on a lead battery (old one used in a car before 65Ah) during consuming its power.
I have connected one of these 12V -> 230V "power inverter" with a 35 W lamp attached to it.

I come up with a voltage decay curve like the one attached.

This is just one excample for a jump. I have seen them from time to time. No changes in load or anything happend at that timepoint.
What is this? Error in measurment or strange battery. Either what the reason is: How can one explain that? How to fix that?

The voltage was measured by a voltage divider build of 3 4k7 resistors and 5V reference.

PLUS(bat) --- 4k7 --- 4k7 --#A0# - 4k7 --- GND

Thx for some infos or ideas.

The code is pretty simple:

long i = 0;
void setup() {

void loop() {
int sensorV1 = analogRead(A0);
int sensorV2 = analogRead(A1);


i = i + 1 ;  


You can't actually say that the load didn't vary at that point - the inverter circuitry and its microcontroller will respond
to the changes in battery voltage (and perhaps internal parameters like temperature) and might cause a change in its
operation mode whenever it sees fit - this is outside your control.

Having said that it looks like a big jump, which is indeed rather hard to explain.  One thing that can cause a sudden increase
in battery efficiency is when bubbles forming on the plates detach and rise to the surface - jogging a battery can trigger lots
of bubbles loose.  However bubbles form during charging rather than discharge (but could hang around for a while).

Another possibility is a poor connection to the battery moved and suddenly got better, reducing voltage drops.
[ I won't respond to messages, use the forum please ]


300W is a big inverter. I would bet the glitches are due to its switching, and especially
if the leads to the battery are longer than 4" and anything other than 10-gauge wire.
Secondarily, the old battery just can't handle it.

Thanks for your replys.

So here are some thoughts:

- I somehow do not have the impression that the thing comes from the Inverter
To rule that out I will connect just a high power resistor without anything else.

- I have also have seen those jumps during charging (inverter not connected)
But here I thought they are intended from the charger to prevent sulfitication...

- I charged the battery before.

I have been reading on voltage measuring accurancy. So perhaps this thing is not a jump in 12V but a drop in 5V (USB supply)

I will also have a look on this (hopefully with internal ref voltage)

Sadly it turns out that this battery is probably waisted. I could get only about 10% of the indicated capacity. This is a rough estimation because I was not jet measuring current.  But the magnitute indicates to not put a lot more effort in this. :-(


If the battery was ever allowed to fall below 10V it's trashed.  You can't get away with that on a lead-acid battery.  If its
been stored without regular trickle-charging its also trashed (sulfated).

However you may get quite a bit of capacity back if you trickle-charge it at a few amps for several days in a warm place - much of
the sulfate deposits can shift from the plates due to bubbles and warmer temperatures - thus uncovering undamaged
plate.  But don't expect more than about 25 or 30% capacity in the best case.

Car batteries are not deep-cycle (if you deep cycle them they lose capacity in a few dozen cycles).  However they can produce
very high currents (when new!).
[ I won't respond to messages, use the forum please ]

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