Hot swap battery system

[mrballistic]

Hi Guys,

Another thing I'd like to work on is a hot swap battery system for an embedded PC. I have an embedded PC that I need to run off batteries and have 2 Li-Ion packs (with SMBus). I need the PC running and would like to be able to hot swap the batteries in situ so that I can replace one with a charged version when it runs low, but be able to keep the PC system running.

I have seen some kits from Ocean Server but they seem so expensive, but seem to have other functions such as charging built in, which I'm not interested in.

Any ideas???

 

[shortbus=]

A Super Cap? **broken link removed**

 

[Gary B]

Two batteries in parallel and diode isolated from each other. Add a disconnect switch for each battery to prevent arcing at the terminal. A MOV across the disconnect switch contacts will prevent damage to the switch contacts if opened under heavy load. Of course, if that happens, you have just defeated your purpose by disconnecting the wrong stack.

Switch a battery off and remove to recharge or replace. Turn it back on for normal operation before doing the same thing to the other battery. The diodes prevent the two batteries from self-discharging each other. This way you have maximum flexibility; just don’t let both batteries run dry at the same time.

 

[mrballistic]

Thanks for that, but I have a further question regarding switching between the packs. The nominal voltage is 14.4V and I am using SMPS converter supply circuit(s) to supply the other voltages (12V, 8V, 5V, 3V) but was wondering what effects would be caused by having the 2 batteries connected because they will be at different voltages to each other. Although they would be separated by diodes to prevent them from trying to charge each other would I be better off living with them at different votlages and changing them when they run flat or having some kind of switch (soft) to flick between the 2 packs.
By the way the system is able to run off one pack quite comfortably.

 

[Noggin]

If you simply diode-or the batteries together, both will run flat at approximately the same time. How much current does your system draw? Can a simple coin cell keep it alive (perhaps in a sleep/idle mode) while you change the battery?

 

[Gary B]

When I say two batteries in parallel, I am talking of a working battery and a freshly recharged replacement of the same voltage. They can be on line together for a while if you want, or just long enough to switch over to the new battery. The diodes ensure that no current will flow in to or out of the lower voltage battery.

What you would normally do is connect a fresh battery and turn on its disconnect switch. You would then turn off the old battery’s disconnect switch (which, since the new stack is providing the current, won’t have an arcing issue) and disconnect it for recharging. If you are going out of town for a while, you can leave both batteries connected so it will still be up when you get back.

 

[kinarfi]

assuming your using a laptop that can run off of a plugged in power supply that also will also charge the battery, and assuming you need to do this because you don't ac power at the computer sight, make a power pack that delivers the 12 vdc to the plug in and then pull and replace the battery while the 12 volts is plugged in. To test this idea, I just pulled my battery and every is still working.
Maybe you could just run the computer off a 12 battery and use the computers battery to swap 12 volt batteries.
Kinarfi