Connecting auxiliary battery to automotive battery

[convoluted]

Hello. I'm trying to devise a clever method to power a PC without draining the main car battery when the car is off. The idea is that when the car turns off, the PC will enter sleep mode and discharge only the auxiliary battery. When the car is on, both batteries should be charging. Ideally the auxiliary battery should be a smaller, cheaper battery.

I've originally considered placing relays between the two batteries, which would be controlled via an automotive microcontroller. My concern is that you cannot mismatch two different types of batteries (including age/discharge rates, etc). Can anyone confirm this? I've also looked into diode and solenoid isolators (article: Battery Isolators). What would be the best way to approach this project? Thanks

 

[k7elp60]

I think using a diode battery isolator is the wrong way to go, as the normal car battery may not get fully charged.
I assume now you are using the cigarette lighter socket to power the laptop. There is a way to use the cigarette lighter socket to charge another battery that will run the laptop. A number of factors need to be considered. 1. The maximum length of time you want to use the laptop on the auxillary battery with the engine off. 2. The current the laptop requires at the cigarette lighter socket in amps, and 3. What the measured voltage at the cigarette lighter socket is with the laptop running and the engine running. With these three values I can recommend a solution to run your laptop on an auxillary battery that is charged by the engine and when the engine is off will run the laptop.
I have done this very thing on 3 different vehicles to run electronic equipment from the auxillary battery.

 

[Blars]

Motorhomes almost always have a relay-based battery isolator between the chassis and house batteries. While both are lead-acid 12v, they are not at all matched in capacity.

 

[convoluted]

Thanks k7elp60, any suggestions you could provide would be great. The PC at the moment is wired directly to the 12V battery, would be best to steer clear from inverters and such. The PC is specifically made for automotive applications, hence it operates off of 12V and draws 7 amps on full operation. On sleep mode it draws around 100-200mA, so I suspect I may get a week or two running in sleep mode off the auxiliary battery (assuming around 50Ah, but doesn't need to be this high). I have a DC to DC converter that is between the 12V battery and PC, so any fluctuations in voltages (on starting car and such) will be smoothed on the output of the converter.

Blars, great to hear that the batteries need not be matched. Would simple mechanical relays do the job?

 

[k7elp60]

Thanks k7elp60, any suggestions you could provide would be great. The PC at the moment is wired directly to the 12V battery, would be best to steer clear from inverters and such. The PC is specifically made for automotive applications, hence it operates off of 12V and draws 7 amps on full operation. On sleep mode it draws around 100-200mA, so I suspect I may get a week or two running in sleep mode off the auxiliary battery (assuming around 50Ah, but doesn't need to be this high). I have a DC to DC converter that is between the 12V battery and PC, so any fluctuations in voltages (on starting car and such) will be smoothed on the output of the converter.

Blars, great to hear that the batteries need not be matched. Would simple mechanical relays do the job?

Lead-acid batterys, either the flooded type or the gel-cell type full capacity is based on the 20 hour rate. So for example you wanted to run the computer for 20 hours @ 7amps it would take a battery with a 140 amp/hour rate. I think you are pretty close on you calulations for the sleep mode and the 50AH. I think your best approach is an auxillary battery that is charged from the engine alternator. I'm not current with the latest technology with battery isolators. I do know that the diode isolators do have a voltage drop and the alternator has to be reset to a slightly higher charge voltage to over come this drop.

 

[MikeMl]

Your biggest problem comes when you connect a depleted (deep-cycle) Aux battery across the fully-charged starting battery. You must devise a method of limiting the initial charging current to the Aux battery, both to protect the Aux battery itself (it has a max charge rate spec), the alternator, which overheats if connected to a mostly discharged battery, an to protect the wiring.

This is a tough problem...

 

[tcmtech]

Reverse your battery system so that the starting battery is the isolated one that way its stays charged up even if the rest of the system gets drained down.