Charging AGM battery from car

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Old 10-12-19, 05:12 PM
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Charging AGM battery from car

Hi guys, I decided that I'd want to leave my deep cycle 75AH battery in my car, and make something that will enable me to charge it. At first I thought that I could simply connect it to the car's alternator, but I'm not sure it would be very healthy having the car "charging" all the time, I'd also use a diode of course so it will only be able to go in one direction.

I could still use the same principal and simply have a switch that will enable me to charge it only when I need to, and that sounds fairly reasonable to me, I'm just not sure how well that's going to be.

One more solution I thought of was getting an inverter similar to this for example: https://www.aliexpress.com/item/3286...chweb201603_55

That seems fairly reliable, then I could simply connect it to the car socket and charge my battery using my 220V smart charger I have, but how good will that actually be? Will that even work?...

Would love to hear some solutions and ideas, I believe it's usually more common in 4WD cars etc. Thanks!
 
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Old 10-12-19, 05:17 PM
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Using a diode in series will automatically drop .7v from the charging circuit.

Since that battery and your car battery are different.... they will charge at different rates. I would just connect it directly for several driving hours. If you only do short trips.... you could use a continuous duty solenoid to connect it when charging is wanted. When the engine wasn't running... the solenoid would be open.
 
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Old 10-12-19, 05:51 PM
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Thanks for the comment! I never knew that diode drops the voltage, I guess that because the car voltage when running would probably be around 13.8-14.2V dropping another .7V would not be very good for the AGM battery?

Should I simply connect the AGM battery to the car battery directly?
I mostly do short trips, so the continues duty solenoid sounds like a great option, can you elaborate more on that?
Since I'm not all that familiar with that, can you please explain a bit about how I should wire it? Is there a specific solenoid I need to get? What would be the appropriate wire gause I should use? Any other parts I would need?

Also, does that mean it will charge only when the engine is running? Thanks
 
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Old 10-12-19, 06:56 PM
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In my boat I have a switch to select between two batteries. I can select bat1, bat2, both or OFF.
 
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Old 10-12-19, 06:58 PM
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Not sure I understood, is it related to powering the boat or charging the boat? Can you please elaborate more on that?
Thanks
 
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Old 10-12-19, 09:46 PM
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I like Cole Hersee relays for their reliability. In the link below you'll see others. Any solenoid rated for continuous duty will work. One terminal goes to the car battery. One terminal goes to the aux battery. The active terminal on the solenoid goes to the fusebox. It needs to be connected with an ACC circuit that only comes on when the key is in ignition but goes dead during start. You don't want the aux battery to be active for starting. I recommend a 100A fuse at the battery. It's called a catastrophe fuse and is a good idea.

I use AGU glass fuses and fuseholders. These new circuit breakers are ok too.
100A protector
Cole Hersee solenoid

Name:  car_aux_charge.jpg
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Old 10-13-19, 01:15 AM
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Thanks a lot Pete! That really looks great.
Since I can mostly buy on aliexpress/ebay, can you please tell me if one of the following solenoids would do the job in your opinion?

This looks like the solenoid you recommended:
https://www.aliexpress.com/item/4000...chweb201603_55

But it doesn't have any reviews at all, so I usually always try to find something with good reviews.

https://www.aliexpress.com/item/3288...chweb201603_55

https://www.aliexpress.com/item/4000...chweb201603_55

These looks rather fine, what do you think?

Also, would a 100A really be good? I thought of using something at around 30A fuse and I thought that even that's a bit too much, 100A seems like way too much for my 75 AH battery isn't it?
 
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Old 10-13-19, 01:15 PM
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I would use the item in the second or third link you posted. Both the same just quantity one or two. Getting two might be a good idea. The first part you linked to is completely different.
The 100A fuse is only there to protect the wiring from shorts to ground..... nothing more.
You can use a smaller fuse. Maybe a 50A.
 
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Old 10-13-19, 03:20 PM
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And is there a fuse I can use to protect the battery from high amps?
Also, since we're not using a diode, is there anything to worry about the car battery draining the AGM battery somehow? I'm not all that familiar with that stuff, so sorry for the dumb question, when messing with such things you should always make sure you understand what you're doing.

Another question about the actual charging, is there any way to control the charging? AGM batteries usually requires float charging or stage charging starting at high amperage and slowly lowering the amperage until the battery is full. Would that be ok for the AGM battery?

Thanks
 
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Old 10-13-19, 03:28 PM
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When using the car as a charge source..... there is no way regulate the charge just to the aux battery. If the car battery is in fairly good shape.... the car rate should be ok. I wouldn't leave the aux battery on continuous charge. Maybe a day or two every week.

The relays job is to disconnect the aux battery so that it is not active during start or when the vehicle is not charging.
 
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Old 10-13-19, 03:54 PM
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Thanks, in case the AGM battery is full, should I still charge it a day or two every week as it might discharge over time on idle?

Do you have an estimation how long it should take for the AGM battery to charge from around 50%? And at what current will it charge? My car battery is fairly new, been using it for a year now.
 
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Old 10-14-19, 07:54 AM
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The AGM self-disharge is low. And, it is best not to store AGM in engine compartments. I don't understand why your battery would ever head for a car??
 
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Old 10-14-19, 01:38 PM
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I don't think you understood correctly, the AGM is not related to the engine compartments.
 
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