Charging SLA from auto battery

Far from home and/or electricity one may have access to a vehicle.  On a recent trip my 7ah battery lasted the entire first day, 9 hours, and about 2 hours into the next day until it hit the low voltage warning on the FF/GPS.

It seems to me that one could easily charge a "depleted" 12v/7ah SLA battery from an automotive 12v/70ah battery by connecting positive to positive and negative to negative.  The automotive battery would force power into the SLA until the two voltages equalized.

Are there other issues to consider? 

Damage to the cells due to overcharging....

Seems it would be safer to buy an inverter and use your regular charger with auto-shutoff.


-Spot-

you could do it, but i ditto on what spot said

If you did, just don't start the vehicle when your doing it.

How many amps are you drawing on that thing? My 5 ah battery last's much longer then that. I'm going on about 15 hours so far and no low voltage warning at all. On past experience i should have a couple more trips in it.

Damage to the cells due to overcharging....
Seems it would be safer to buy an inverter and use your regular charger with auto-shutoff.

I never realized how cheap auto inverters were - $25 would do it for me.  

How many amps are you drawing on that thing?

650ma, for a theoretical total discharge in 10.77 hours.   It must be drawing less because it lasted 11 hours before the low battery warning at 9V.

650ma, for a theoretical total discharge in 10.77 hours.  

Ok, no wonder. Your drawing 3+ times as much juice as i am...

Keep in mind that the smart chargers that are sold with the batteries we use for sonar are typically rated to deliver a max of 1A charging current. They also usually have an auto shut off feature to prevent excessive gassing and physical damage.

Charging directly from a car battery will likely ruin or reduce your sonar battery to a paperweight.

Using an inverter and the smart charger will eliminate the inevitable melt down.

650ma, for a theoretical total discharge in 10.77 hours.   It must be drawing less because it lasted 11 hours before the low battery warning at 9V.

Is there a way you can reduce the current draw? Decrease back lighting? Turn off GPS functions?

http://www.earthtechproducts.com/p2007.html       Kind of expensive, but if they were waterproof, it would be cool to throw somewere?

Nah, go with their Solarflat if  you go that option.  Check out the 5 Watt 12V version
http://www.brunton.com/product.php?id=615

They overprice on their site, you can find it from dealers cheaper.

Nah, go with their Solarflat if  you go that option.  Check out the 5 Watt 12V version
http://www.brunton.com/product.php?id=615

They overprice on their site, you can find it from dealers cheaper.

Great features but it would be a little cumbersome to store and deploy.
I like the idea of having a flexible panel that can be bungied down to your yak.

http://www.brunton.com/product.php?id=256

I ordered a small inverter to keep in the truck - $27 with shipping.  Thanks.  

Is there a way you can reduce the current draw? Decrease back lighting? Turn off GPS functions?

No need, though I might be able to turn off the GPS, and I do have the backlight on maximum.  I am not a camper, so 10 hours of function is plenty.  It seems to take about half as long to charge as discharge.  I could have pulled the battery and charged overnight in the hotel room, but I forgot.  Unfortunately, an inverter is not going to help my memory, but it will negate having to pull and replace the battery when traveling. 

I realize Demonick went with the invertor, but I thought it was worth mentioning that you can find 5watt 12v Brunton Solarflat for $60 in that big store in Lacey 

I'm gonna pick one up in a few months.  The comparable solar roll (providing 4.5watts) is more than twice the cost.  Although it's cool to be able to roll it up, but if you're not hiking, you could keep it strapped under the bungies on the front or back of your yak, which is where it would need to be for charging anyway.

Unless I find some type of convertor, I'll likely pick up a small 5v output foldable/packable panel for charging phone and camera (USB output)  These run around $70

How many amps are you drawing on that thing?

650ma, for a theoretical total discharge in 10.77 hours.   It must be drawing less because it lasted 11 hours before the low battery warning at 9V.

Check the specs.  For instance ... http://www.empirescientific.com/SLA%207-12.pdf

Page 1 has a chart of voltage vs. discharge time or various current loads.

For instance, this particular battery should last ~11 hours with a 0.63A load before it discharges to ~10.5V.  The curve doesn't extend any further, but you can see from the shapes of the curve that they go downhill fast.

Regarding those 5W solar chargers.  Those ones charge at a 350 mA rate.  A fully discharged 7Ah battery would take 20 hours to fully charge.

-Allen