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How to make a cheap isolated dual-battery setup for $50

john61ct

Adventurer
Adjustable setpoints are a big point in its favour, may pick one up for testing.

The LVD cutoff could always be controlled by an external relay / BMS.
 

dwh

Tail-End Charlie
Note A: When the battery is charging through CH3 with normal operation (not through Ignition or override setting), the charger will charge for 3 minutes, it will then rest for 5 seconds to verify the Input voltage. During the 5 seconds rest period, if the measured voltage is > Normal Under Voltage Recovery Voltage (> 12.8Vdc on a 12V system or > 25.6Vdc on a 24V system), the unit will continue the charging cycle through CH3. If the measured voltage on CH3 is < Normal Under Voltage Recovery Voltage (< 12.8Vdc on a 12V system or < 25.6Vdc on a 24V system), the unit will switch back to charge from CH2 (PV) until CH3 returns to Normal Start-Up Voltage (13.2V on a 12V System and 26.4V on a 24V system).

Also, during the 3 minutes charging process, if the measured terminal voltage drops below the Under Voltage de-rate voltage (11.5V for a 12V system, 23V for a 24V system), the charger will start to de-rated the output current. This function is used to compensate for the use of long or thin wire between the Input Battery System and the unit terminals. If the input voltage continues to drop below the Under-Voltage Shutdown Voltage (10.5V for a 12V system and 21V for a 24V system), the charging process will terminate and the unit will switch back to charge from CH2 (PV). It will only switch back to CH3 if the CH3 voltage returns to 13.2V on a 12V system or 26.4V on a 24V system



Voltage on alternator input has to be greater than 12.8v. Less than that and it switches to solar and doesn't switch back until alternator input exceeds 13.2v.
 

WVI

Adventurer
Good Day Folks.
I'm looking at adding a simple system to my van for trips.
Probably on the order of Post #4 but with an electronic battery monitor/isolator/ disconnect.
About the only thing I will be running are a roof vent/fan, a small fridge, a couple of led lights and a usb plug or two.
What I'm wondering is, would this work out if i wish to put a cordless tool charger in (dewalt) and leave a battery on it to stay charged, just to have it handy when I need it? I could run a small inverter to power it.

Thanks in advance for any thoughts provided.
 

dwh

Tail-End Charlie
Works fine. I charge my Ryobi 6ah 18v lithium batteries with a charger that plugs into 12v. A 120v charger into my inverter would probably charge the battery faster, but I haven't tried it.

I don't leave it plugged in though. Lithiums don't need a constant top off. When they're full they're full. And I've smoked chargers before so I'm not real trusting of leaving it plugged in when unattended.
 

WVI

Adventurer
Thanks for the advice.
I kind of wo Derek about bleed off on the small batteries due to one dying when I left it in the 1/4 impact.
Since then I take them out of the tool.

I will do some more reading on the DC-DC chargers. I like the idea of them but have no experience with them, and am not sure the dollar output for it wouldn't be better put towards a small so,ar to add to it down the road.
 

llamalander

Active member
A small inverter can be useful for charging batteries that need a wall plug, like toothbrushes, laptops or tool batteries, or inflatable mattresses, but not as efficient--which may be a concern if you don't have more storage than you need. Auto tool battery chargers seem to take 30-50% longer to charge, but can be worth it if you use them often. My truck charges batteries for work, but when I'm camping--er, overlanding--I use an 18v. flashlight and usb charger daily, and keep a few tools in the truck should I need them. I have both an inverter and a dc tool charger, the latter I use 10 times more often, and it cost 2/3 of the inverter.
 

WVI

Adventurer
I will have to look at a dc tool charger. It's not really a big deal. I only really use it for my little dewalt impact which is a big time saver and helps with the old joints.
 

mobydick 11

Active member
OK so I did the dual set up on my 09 GMC today . the good cost out of pocket was 60 bucks and the 09 charges at 14.7 volts . also there is already a fuse between the battery and the main fuse block .so if you hook on the other side of that you have one fuse already .come to think about if maybe I had better switch sides . but this is what I had lift over from other projects that I used . a battery, a new 200 amp solenoid ,a tie down kit and the toggle switch .Just bought the cables to run between battery . my set up is simple in that when you want to charge the battery from the truck you throw the toggle to the on position . this cold also be used as a booster I think . the bad when I was hooking up the second battery ,doing the positive before the negative bam there is a arc . lesson learned if the toggle is in the on position you are grounded through the main battery . Shoot I would have to be using a snap on wrench .
 

WVI

Adventurer
Boy that could smart!
I'm still leaning toward an electronic isolater that will sense when it needs to be charged.
My memory isn't up to a toggle switch 😁
 

mobydick 11

Active member
Well I have a complete kit for adding a second battery to another project . But after reading this thread I wanted to have some fun and try. this morning I am going to turn the toggle switch around ,so if it gets bumped it will be in the off position . and change the cable to the other side of the fuse .
 

kevman

Observer
I'm assuming this has already been covered but my internet skills are letting me down today. What kind of terminal should be used for connecting the 2nd battery to the starter battery? My starter battery currently has a set of post clamps (I don't know the actual name) but since there is nowhere to bolt down my second battery cable/fuse I assume I'll need to replace this. Can I get a post and thread mount that clamps to the battery? That way I wouldn't need to modify the existing cable terminals. Or am I better off with crimping a set of lugs onto the existing cables and using some sort of post to thread adapter?
 

69XS29L

New member
It sounds like you are looking for something like a "mil spec" battery terminal available on Amazon. Allowing you to bolt multiple wires to the same battery post. I've used the Audew and CrimpSupply brands, prefer the latter but they are mostly the same. I've also bought them in better quality from electrical retailers specializing in marine and heavy equipment. You'll need terminals, wrap and crimper for the wire gauge you are working with. TemCo is a great place to purchase this stuff, they have an Amazon storefront. Their copper wire and tinned terminals are American made for the most part and more reasonable than the Chinese sheeite everybody else sells.
 
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