Monitoring battery voltage w/ BSII

jlosaw

Hi...

I have a BSII, and I want to monitor the voltage of a battery. I need help with the programing and the circuitry.

I race RC cars that run battery packs with 4 sub-C size cells. After the race, the batteries get put in a battery tray that connects each cell to its own capacitor to dump the pack and equalize the voltage in the cell. (See pic for images of the battery and the discharge tray.) The battery manufacturers recomend that the voltage in the cells be drained to not less than .9V/cell. So, I want to use the stamp to monitor the voltage in each cell as it discharges, and to break the connection between the battery and the capacitor when the voltage reaches .9.

Does anyone know if·this is possible, and how can I do it. Do I need an analog to digital converter to read the battery voltage and a relay for the battery/capacitor connection? The cells are 3300mAh so I think the current is too high to put throught the stamp.

Any help is greatly appreciated.

Thanks
Jeremy

Here is a link to the system I am trying to emulate, if it helps.

http://www.teamnovak.com/products/dischargers/smart.html

K de Jong

Hi Jeremy,

If I read your message right you have in fact two questions:

1 how do I monitor battery voltage with my BS2

2 how do I swith of discharge current if the voltage drops too low

To monitor the battery voltage the best, and most convinient, way would be to use an analog to digital converter. There are many different types on the market, I normaly use the TLC2543 which has 11 (eleven !!) channels with 12 bit resolution. This is overkill most of the time but there are more.

For disconnecting from discharge current you can use a relay. You will need a transitor, or darlingtong, to switch the relay from the Stamp. Remember a diode for your relay.

Finally, a verry simple way to prevent discharging battery's to go too low in voltage is to put some diodes is series to your discharger. Each diode has a voltage drop of about 0.7 Volts over it. So if you have 4 battery's x 0.9 Volt = 3.6 Volt minimum.

Put 3.6 Volts minum / 0.7 (diodes) = 5 diodes in series and your batterys are protected .

Regards and good luck,

Klaus

Dave Paton

Jeremy-

I'm in the process of building a very similar box to that you're looking for, for my Speedmerchant Rev3 and it's stable of packs. I'm using a multichannel ADC and a high current low side switch to monitor each cell individually and switch them to various levels of discharge as required. Right now I'm under contract for the development effort, so I can't share the exact design here, but I can tell you to look at Analog Devices for octal ADCs and Zetex for parts to build high current, low RDSon, logic-driven low side switches. The code is actually really simple if all you want to do is discharge to 0.9V/cell. My box is also monitoring current, and doing a little magic to discharge the packs at race levels of current, and with rate changes like in a race, not just an arbitrary 1A or 10A (I've measured some runtime improvements and more reliable IR). Final question: What kind of cells are those?

-dave

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jlosaw

Hi Dave and Klaus...thanks for the help

yeah...that would be sweet to have an active battery discharge cycle...especially if you had a map of the current over the course of a lap, you could cycle the batteries as per race condition...if i can get the simple one working that would be the next step surely.

couple of ?s for you Dave...

is an octal ADC just an 8 IO pin ADC?
what is Zetex?

i see you do the on-road stuff...i'd much rather do that than stupid ovals, but that is whats popular in the upsate ny area.

the cells are some used pro-match GP3300s...i just got back into racing after a 6 year hiatus to get meself some edjamacashun, so i needed to upgrade fast and cheap from the 1700s that i had, so i went used.

KenM

For your battery cells, if I correctly understand, you are only interested if the battery cell voltage is above 0.9 or not, is this correct?

If you need only to know above/below a certain threshold, one suggestion is to use an comparitor (LM311) with the reference terminal set at 0.9 volts via a voltage divider network and a stable voltage supply.

The comparitor can also use positive feedback to avoid oscillations at the 0.9 volt "cross over" point.

ken

vir-technologies

Jeremy,

All the BSII needs is an ADC, and you can charge and monitor batteries, and improve the performance of the RC vehicle. A 8-12 bit ADC will work fine, and code is fairly simple. Below is a link to battery management source code I have designed and used over the past several years, and the core functions have been used in NASA since the early 1990's.

Please review the kit listed here, vir-inc.com/software.html, for circuits and source code which can be downloaded with the IPMCharge code - PBASIC SDK.

This simple circuit could just snaps on to the BSII Carrier Board, and you have a complete solution for rechargeable batteries.

I caution you to not look at battery voltage alone to determine low or high battery levels, as this does not provide any data on the efficiency of the cells to power the device. This applies to wireless phones, RC vehicles or notebook PCs.

Regards,

Steve Halla