I have a small 12V lawn and garden battery that I can charge at home and take car camping for a couple of nights. I'm going on a longer trip and would like to just plug it into the spare 12V cigarette lighter-type outlet in the hatchback of my car and let it charge while I'm driving, then unplug and carry it to my campsite for lights/radio. Is there any problem with charging it in this way? Will it overcharge the battery? or affect the car in any way? Thanks.
I think you will be ok with that arrangement. Does the lighter plug have a fuse inside it? Just in case there should be a problem occure. But since it is basically a lead acid battery as your car uses, it sould do just fine.
Hi, Marco. The 12V lawn and garden battery is a deep-discharge lead-acid battery. It should do just fine with your alternator voltage (about 13.8 - 14.2V) and not overcharge.
You'll have a minor problem with your basic setup. When you plug in your deeply discharged battery, the initial current will be pretty high, and will probably pop the cigarette lighter fuse (I'm assuming it's fused, of course).
The easiest thing to do would be to just grab some power resistors out of your junkbox, and set it up so you've got one or two ohms of series resistance (make sure you use power resistors to be able to dissipate
50W or so. This should keep you from popping fuses.
If you want, you can then monitor your recharging battery voltage. When it gets above 13.2V or so, you should be able to short across the resistors, and directly charge the rest of the way.
If you really wanted to get tricksy, you could throw in a relay to automate the switchover like this (view in fixed font or M$ Notepad):
The neat thing about this little trick is that DC relays actually pull in at somewhere around 60% to 80% of their nominal voltage. You could do this on the cheap by scrounging a good automotive relay with a normally open contact (make sure it's capable of switching at least
30A). Use a DMM to measure the coil resistance. Then either use a power potentiometer, or substitute series resistors such that the relay just snaps in at a charging battery voltage of 13.2V or so.
Relays are remarkable beasties in some ways. They have built-in hysteresis (preventing intermittents if the voltage is right on the edge, then drops just a bit when you turn on the headlights), built-in noise filtering (an automotive relay will stay closed for several ms. if power is interrupted or sags), and it's hysteresis points are fairly stable over temperature.
By the way, as a practical safety matter, please remember that the battery is fairly heavy, and also contains a strong acid which could hurt someone if it spills. Be sure to physically secure the battery , and also to cover the terminals, if you're driving with it. But do not completely enclose the battery -- generation of hydrogen gas during charging can cause even bigger problems.
Greetings Marco, Chris had a bunch of good advice but I would like to add a little to it. When the battery is charging it not only outgasses hydrogen. It also can release acid fumes. These fumes are very corrosive over time and you may not realize until things start to rust in your car that there is a problem. I added an extra battery to a van years ago. It was a Ford Econoline van with the engine and battery basically in the passenger compartment. The battery box was under the driver's seat. It was completely sealed from the passenger compartment but had holes through the floor to vent any gasses. When I added an extra battery I used a stout plastic box with a lid that fit tight to hold the battery. I used a barbed hose fitting in the side of the box and ran a
5/16" I.D. vinyl hose from the box through a hole drilled into the engine compartment. You may want to consider using a Rubbermaid box or similar and run a hose outside the car to vent it. Also available, but kinda spendy (probably 20 bucks), are plastic battery boxes for marine use so you can put a battery safely in your open boat. One of these could be mounted under the hood. Good luck! Eric
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