SparkFun Forums

[science_geek]

i need some sort of circuit or ic that can monitor 24v lead acid batteries and send the amount of charge left to a basic stamp 2 which will decode the info and send it to my display, the display is from 4d systems and the main reason im using the bs2 is because of how much it costs compared to the display and i dont want to fry the display

[bigglez]

Greetings (No First Name Supplied),

i need some sort of circuit or ic that can monitor 24v lead acid batteries and send the amount of charge left to a basic stamp 2

Much easier said that done! There is a flaw in your
reasoning - to know the remaining charge in a
cell you also need to know that cell's capacity and
whether it was fully charged when you started.

The usual method of measuring cell drain is to
monitor the current and use a coulomb counter
to integrate the charge over time.

This circuitry is not readily available in a single
IC as you are operating from a 24V battery.
Most battery gauge ICs are geared to low
voltage batteries found in cellphones and
video cameras.

Here's some background reading to get you
started, and a link to one vendor's offering.

Comments Welcome!

[science_geek]

so is there any other way i can tell if my batteries on my electric go kart are about dead and i should return home or am i just set at guestimation

[bigglez]

Greetings (Still No First Name Supplied),

so is there any other way i can tell if my batteries on my electric go kart are about dead and i should return home or am i just set at guestimation(sic)

Possibly you can monitor the battery voltage, which
falls slowly over the discharge time. The rate of change
of this curve increases as the charge is nearly depleted
(as noted in the link above...).

Comments Welcome!

[lyndon]

As bigglez said, there is no simple, accurate way other than a gas gauge IC and as far as I know, even those aren't very accurate because battery characteristics change over time.

A simple, not very accurate way is to monitor the no-load voltage of the battery. Perhaps a comparator set to a voltage limit specific to that battery that indicates the charge is getting low. The problem is that the voltage difference between a good charge and low charge is very small.

Better methods are coulomb counting as mentioned in the earlier post or impedance measurement. Impedance measurement is a good way to tell that the battery is getting low, but it works best on a cell-by-cell basis. It's not that difficult to do, but it's not a simple, single chip solution either. I also wouldn't recommend attempting the design if you don't already have experience with analog circuitry.

Actually I just noticed that he mentioned monitoring the rate of change of the discharge curve. The change in the discharge slope is due to the impedance changing as the battery is depleted. This would be a simpler way than measuring the impedance directly.