2 Batteries/battery isolation...
 

2009 Honda Accord
EX-L, 6cylinder
All leather, only 35k on it...
Nothing extra to drain the battery, like 3rd party audio components, wacky lights, or 12vDC doomsday devices...


-I replaced the original/factory battery after 2 years with a 750A crank (original didn't seem to hold a charge-especially when cold, would not start). This was in April 2012, or ~18 months ago.
-I've since ran the car once every week or 2, and now need to replace that battery...
-Again, worst when cold (understandable). I have one of those portable battery starters from Sears (Diehard 1150 model), and LOVE it, as it has saved me many times.

So this gets me thinking about adding a second battery, and using a battery isolator (charges both, drains one when off, uses the other to crank/start.

It isn't that there aren't devices out there, I get that... What I don't know is which ones to buy. For example, is a unit that handles 200A going to do the job, or could I get away with a smaller/cheaper 120A model?

The slot for the battery won't let me put a nice, big one in, so I think I'm restricted there, but I could probably find room under the back seat, or in the trunk, to mount a second battery...

Anyway, there are mechanical and electronic units, and I'm really curious what others have done to install such a system so common in RVs and larger trucks...

TIA!

pat
:)

I moved your thread to the audio/visual section, as there are a couple of knowledgeable electronics guys in this section. It may take a short while before they respond though.

I have a 300 amp Isolater on my car. A PAC. I have no problems out of it. however I am thinking that you may solve your problem but you are not fixing the issue. the up and down drain of the main battery is going to kill the cells in that battery.

I would suggest using a test light or clamp meter to find what your major drain is. If this would be something you would rather do. let me know and I will go more into detail on how to do it. It is fairly easy, just time consuming.

Neophyte,

Thanks for writing. A battery isolator, as I understand it, is a pair of relays that keep the 2 batteries isolated from each other when the ignition is off, and puts them in parallel when the ignition is on. One battery might be considered the 'primary', as it would keep the car's electronics 'ON' while the ignition is off (if that makes sense).

I question what happens, though, when the primary is dead, and you connect them together to start the car. Would the isolator know to not connect the dead one until charging started?

I have a VOM, which I would imagine would be put across a fuse's contacts (with the fuze removed, or a blown fuse inserted to provide easier contacts while measuring current), correct? As we're looking for very low amounts of current (ignition in OFF position), I would think a clamp-on would not only be expensive (Sears has them for $50 here), but maybe impossible to use since most all the wiring is bundled together and hard to separate the wires. Plus, if the currents are really low, a clamp-on isn't going to have the sensitivity and accuracy of an inline ammeter. But you make me wonder just how big a draw I might be looking for... hmmm...

But speaking of devices, with a battery isolator as an option, I've also seen on the market the solid state ones... I don't think I know enough yet to go out and know how many amp unit is needed, where to mount, where to mount second battery, etc.

Could it just be not running the engine for a week or 2 is enough to normally make a car not start? How long does it take a battery to age to the point where this is an issue?

I hope I can bring my battery to be checked to see if a cell is burnt out (will never be 12vdc, as a reult, or something else..

Oh, and while we're on the subject, with such a small cavity to put the battery in, what is the absolute BEST battery I can buy? Or brand... ?

TIA!!!

pat
:)

welcome!

since your issue is due to the vehicle sitting for extended periods of time, you want a "battery tender". a small charger that keeps the battery "topped off" while sitting. the downside is this means the car needs to be parked near an outlet and normally connected. another option is a solar panel battery charger that sits outside or in the rear window (if the car is normally parked outside). this charger will keep the battery topped off as well (if properly sized). i have used solar panel chargers on boats so they are charged every day without shore power.

while sitting disconnected a battery will still self-discharge resulting in lower resting voltage over time.

when the drained battery is paralleled with the stronger battery, it will pull the stronger battery down with it. both batteries will float at the same voltage when connected together.

the best solution is to address draining of the primary battery. a battery tender is only $40-70 and is commonly used on classic cars that are not driven but on weekends and nice days.
Home - Batterytender.com


when looking for a short you remove the fuse then measure DC current through the digital multi-meter. Take note the car should be off since the DMM will be limited to 10A or less. This is fine for current drain measurements. Also note most meters require you move the leads for current vs voltage. don't mess that up or you blow the fuse in the meter. you remove one fuse at a time and note the draw. a few mA is ok, hundreds of mA is the sign of a draw and should be verified - may be normal. more than 500mA is above normal.

also, an isolator is nothing more than a high amperage relay. as soon as isolator gets a signal it connects the batteries, and as KHA stated, it will immediately pull both batteries to the same voltage.

and I missed that your car was sitting for extended time, so it could be simply that you need a tender as kha said. 2 weeks can be enough to drain a battery to the point of not starting the car. the battery tender would be a nice thing to have. they can be mounted in the car and all you would need would be a plug like KHA said. if you buy a new battery the higher the CCA the better it should do in the cold from my understanding. I am not really knowledgeable when it comes to batteries.

if you have a short it will pull a signifigant amount more amperage. you just need the end of the ground and the negative battery post. my wife's mustang had a bad factory amp and I was able to diagnose it with a test light. if you have a nice meter or device for testing amperage great. a $4 test light can tell the difference.