Charging AGM battery from car

[msacco]

Hi, kinda spamming this forum lately

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. 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 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/32867195066.html?spm=a2g0o.productlist.0.0.1c7e67bdN7ABJB&algo_pvid=7bf858c2-8f4e-4df0-9f4a-8b45f57538ae&algo_expid=7bf858c2-8f4e-4df0-9f4a-8b45f57538ae-3&btsid=a2649da2-a392-457f-8420-d9c2d515bab5&ws_ab_test=searchweb0_0,searchweb201602_9,searchweb201603_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!

[johninderby]

They do make 12v to 12v dc chargers.A bit of searching should turn up something cheaper than this one.

https://www.mdsbattery.co.uk/optimate-dc-to-dc-battery-charger.html8

[msacco]

5 minutes ago, johninderby said:

They do make 12v to 12v dc chargers.A bit of searching should turn up something cheaper than this one.

https://www.mdsbattery.co.uk/optimate-dc-to-dc-battery-charger.html8

I tried googling for something like that, but didn't really find any results. I tried searching in aliexpress/ebay but couldn't find anything.

Edited October 12, 2019 by msacco

[discardedastro]

I've actually done just this before at work for a mobile mapping system (laser scanner, camera, and computer) in the back of a Nissan Navara. The whole system could use a dozen amps at peak so we opted to keep it isolated from the vehicle battery but didn't want to have to recharge the battery independent of the vehicle.

The 12V-to-12V charger option is the route to go. Most modern cars won't engage the alternator and pull the float voltage high of the charging level unless they think the vehicle battery needs it, so if you just hook it all up in parallel or similar then you'll find you don't often get the required voltage. The efficiency of the DC-DC approach far outstrips the DC-AC-DC loop.

We went with the Sterling DC-DC chargers - the one we used was https://www.12voltplanet.co.uk/sterling-pro-batt-ultra-waterproof-battery-to-battery-charger-12v12v-60a.html but they have cheaper options.

This can either be attached to the ignition indication circuit in the car (so it only tries to charge when the engine is running) or a separate switch. I'd recommend the ignition approach, though I think we included a separate isolator switch for that in series so it can be switched off entirely (there's also some chunkier isolation switches to let us disconnect the feed busbars from the batteries without having to remove connectors).

You do need suitably chunky cables between your charger and batteries, especially considering voltage drop at the current levels you might anticipate.

If I were trying to hack something up while being incredibly lazy I'd do the inverter+charger route, since that requires no wiring, but it will be slow and generally suck.

[msacco]

6 minutes ago, discardedastro said:

I've actually done just this before at work for a mobile mapping system (laser scanner, camera, and computer) in the back of a Nissan Navara. The whole system could use a dozen amps at peak so we opted to keep it isolated from the vehicle battery but didn't want to have to recharge the battery independent of the vehicle.

The 12V-to-12V charger option is the route to go. Most modern cars won't engage the alternator and pull the float voltage high of the charging level unless they think the vehicle battery needs it, so if you just hook it all up in parallel or similar then you'll find you don't often get the required voltage. The efficiency of the DC-DC approach far outstrips the DC-AC-DC loop.

We went with the Sterling DC-DC chargers - the one we used was https://www.12voltplanet.co.uk/sterling-pro-batt-ultra-waterproof-battery-to-battery-charger-12v12v-60a.html but they have cheaper options.

This can either be attached to the ignition indication circuit in the car (so it only tries to charge when the engine is running) or a separate switch. I'd recommend the ignition approach, though I think we included a separate isolator switch for that in series so it can be switched off entirely (there's also some chunkier isolation switches to let us disconnect the feed busbars from the batteries without having to remove connectors).

You do need suitably chunky cables between your charger and batteries, especially considering voltage drop at the current levels you might anticipate.

If I were trying to hack something up while being incredibly lazy I'd do the inverter+charger route, since that requires no wiring, but it will be slow and generally suck.

Thanks for the comment, since I live in Israel I'd probably need to buy something off aliexpress/ebay or maybe amazon(shipping would be more expensive though).

Do you happen to know anything suitable there I can find for a reasonable price?

Generally, the battery should be fully charged most of the time, I will be using it around once a month, so I don't need to worry much about slow charging etc, but about it being suck I would worry, can you please elaborate more on why it would suck? I'm not looking for an expensive solution, but a rather simple one.

[discardedastro]

Just now, msacco said:

Thanks for the comment, since I live in Israel I'd probably need to buy something off aliexpress/ebay or maybe amazon(shipping would be more expensive though).

Do you happen to know anything suitable there I can find for a reasonable price?

Generally, the battery should be fully charged most of the time, I will be using it around once a month, so I don't need to worry much about slow charging etc, but about it being suck I would worry, can you please elaborate more on why it would suck? I'm not looking for an expensive solution, but a rather simple one.

So each link in that chain will have some losses - DC-to-DC can be 90%+ efficient, but DC-AC-DC is much less efficient. Add to that the fact that your AC charger is just going to assume it has limitless power whereas a well-behaved DC-DC charger will make allowances for the fact its input is imperfect because the car+alternator will be doing various things that they consider normal. The DC-DC stuff has lots of inbuilt logic to protect both batteries and charge/trickle-charge efficiently, whereas the AC charger doesn't know there's a DC battery feeding it and so will just sit there trying to pull power from the AC supply.

You can find Sterling and others on eBay, don't know about aliexpress.

[msacco]

12 minutes ago, discardedastro said:

So each link in that chain will have some losses - DC-to-DC can be 90%+ efficient, but DC-AC-DC is much less efficient. Add to that the fact that your AC charger is just going to assume it has limitless power whereas a well-behaved DC-DC charger will make allowances for the fact its input is imperfect because the car+alternator will be doing various things that they consider normal. The DC-DC stuff has lots of inbuilt logic to protect both batteries and charge/trickle-charge efficiently, whereas the AC charger doesn't know there's a DC battery feeding it and so will just sit there trying to pull power from the AC supply.

You can find Sterling and others on eBay, don't know about aliexpress.

So putting efficiency aside(I really don't need to charge it all that much and often), is there any other reasons? causing any damage to any of the batteries? Not charging the battery correctly?

As for what to search on ebay, I'm sorry but I'm not really sure what keywords to search for, can you tell me what exactly I should search or send me an example for something reasonable?

Thanks

Would something like either of these be sufficient?

https://www.aliexpress.com/item/32844368647.html?spm=a2g0o.productlist.0.0.5b196ae6d22tLp&algo_pvid=df71323b-37ba-42a2-b34e-54c65d0a9792&algo_expid=df71323b-37ba-42a2-b34e-54c65d0a9792-0&btsid=a0d84826-3a6c-43ff-a17f-97282e1e3c96&ws_ab_test=searchweb0_0,searchweb201602_9,searchweb201603_55

https://www.aliexpress.com/item/32915492330.html?spm=a2g0o.productlist.0.0.5b196ae6d22tLp&algo_pvid=b7723cfd-1bb5-4876-b89b-26885f56d09c&algo_expid=b7723cfd-1bb5-4876-b89b-26885f56d09c-15&btsid=d447a05b-6dcb-40ed-9231-5e3d280adef5&ws_ab_test=searchweb0_0,searchweb201602_9,searchweb201603_55

I assume that the 6A current would be a bit low for an AGM battery?

Edited October 13, 2019 by msacco

[JRWASTRO]

Hiya Msacco,

Here is a very robust unit  :  https://www.redarc.com.au/smart-start-bcdc

This will suite your 75Ah deep cycle battery.

Jeremy

Edited October 13, 2019 by JRWASTRO
put in some spaces to get the link to work

[msacco]

2 hours ago, JRWASTRO said:

Hiya Msacco,

Here is a very robust unit  :  https://www.redarc.com.au/smart-start-bcdc

This will suite your 75Ah deep cycle battery.

Jeremy

Thanks, but there's no way I'm going to invest so much in a battery that I bought for $140. I would rather buy 4 batteries for that price.

[discardedastro]

9 hours ago, msacco said:

So putting efficiency aside(I really don't need to charge it all that much and often), is there any other reasons? causing any damage to any of the batteries? Not charging the battery correctly?

As for what to search on ebay, I'm sorry but I'm not really sure what keywords to search for, can you tell me what exactly I should search or send me an example for something reasonable?

Thanks

Would something like either of these be sufficient?

https://www.aliexpress.com/item/32844368647.html?spm=a2g0o.productlist.0.0.5b196ae6d22tLp&algo_pvid=df71323b-37ba-42a2-b34e-54c65d0a9792&algo_expid=df71323b-37ba-42a2-b34e-54c65d0a9792-0&btsid=a0d84826-3a6c-43ff-a17f-97282e1e3c96&ws_ab_test=searchweb0_0,searchweb201602_9,searchweb201603_55

https://www.aliexpress.com/item/32915492330.html?spm=a2g0o.productlist.0.0.5b196ae6d22tLp&algo_pvid=b7723cfd-1bb5-4876-b89b-26885f56d09c&algo_expid=b7723cfd-1bb5-4876-b89b-26885f56d09c-15&btsid=d447a05b-6dcb-40ed-9231-5e3d280adef5&ws_ab_test=searchweb0_0,searchweb201602_9,searchweb201603_55

I assume that the 6A current would be a bit low for an AGM battery?

Those RC battery chargers should work but won't be "automatic" - they'll require you to configure the charger and start it. You'll need to make or buy an XT6 to 12V cable. If you're using a cigarette lighter socket then you'll normally be limited to around 6A anyway.

[msacco]

10 minutes ago, discardedastro said:

Those RC battery chargers should work but won't be "automatic" - they'll require you to configure the charger and start it. You'll need to make or buy an XT6 to 12V cable. If you're using a cigarette lighter socket then you'll normally be limited to around 6A anyway.

Thanks. Currently I believe I'll go with something similar to this:

[johninderby]

Check the specs of the one I linked to. Fully automatic with 7 stage charging and auto shutdown if voltage drops too low.

[msacco]

1 minute ago, johninderby said:

Check the specs of the one I linked to. Fully automatic with 7 stage charging and auto shutdown if voltage drops too low.

From what I've seen it seems to charge at 2A max, I'm not sure how good that would be for the battery, even with 7 stage charging.

[johninderby]

Deep cycle betteries take a charge better if charged slowly over a longer period. 

[msacco]

2 minutes ago, johninderby said:

Deep cycle betteries take a charge better if charged slowly over a longer period. 

I'm not an expect, but the charging should be float charging starting at rather high amps then slowly lowering the amps as far as I'm aware.

[johninderby]

It all depends of if you are charging up a battery from flat or maintaining the battery. Can be a complicated business. If a deep cycle is completely flat 10 to 15A is recommended.

[msacco]

Just now, johninderby said:

It all depends of if you are charging up a battery from flat or maintaining the battery. Can be a complicated business. If a deep cycle is completely flat 10 to 15A is recommended.

Generally most of the time it will be for maintaining, but if the battery reaches around 50% after a day or 2 of use, then I will need to charge it harder.

[JRWASTRO]

Re:

I believe it's usually more common in 4WD cars etc

Yes! But you will have to pay. Cheap Chinese rubbish, in most cases, will not fit the bill!!

23 hours ago, msacco said:

https://www.12voltplanet.co.uk/sterling-pro-batt-ultra-waterproof-battery-to-battery-charger-12v12v-60a.html

The unit above looks excellent.

 

Jeremy.

[Gina]

I wouldn't skimp on battery chargers - batteries can self destruct in spectacular fashion if not treated right!!  Beware batteries can become time bombs in some circumstances!

[Mick J]

We used to have RC toys, charged the toy batteries off the car battery when out and about, over a long day we managed to flatten all the toy batteries and the car battery, stuck in a field late afternoon !!   Someone nice jump started us and we drove around for a while expecting to stall at a junction,  see where I am going with this 😁

Caravans and small boats have circuits/controls for multi batteries, power for essentials and run the lights and TV off another battery, some sort of circuit that balances the charging. This may not be the most helpful post but the last thing you need is everything flat.

[msacco]

1 hour ago, Mick J said:

We used to have RC toys, charged the toy batteries off the car battery when out and about, over a long day we managed to flatten all the toy batteries and the car battery, stuck in a field late afternoon !!   Someone nice jump started us and we drove around for a while expecting to stall at a junction,  see where I am going with this 😁

Caravans and small boats have circuits/controls for multi batteries, power for essentials and run the lights and TV off another battery, some sort of circuit that balances the charging. This may not be the most helpful post but the last thing you need is everything flat.

How did you charge it? If I'll charge it from the car battery, it will be ONLY when the engine is running so the alternator will take care of it.

[Mick J]

2 hours ago, msacco said:

How did you charge it? If I'll charge it from the car battery, it will be ONLY when the engine is running so the alternator will take care of it.

Used an RC charger like you linked to above HERE,  we would just crocodile clip onto the battery and plug the toy's battery in,  did not notice the car battery getting low until packed up and ready to go home.  We used a small petrol generator after.

[msacco]

4 hours ago, Mick J said:

Used an RC charger like you linked to above HERE,  we would just crocodile clip onto the battery and plug the toy's battery in,  did not notice the car battery getting low until packed up and ready to go home.  We used a small petrol generator after.

Well that sounds like a completely different story then, as that shouldn't happen in my case. Obviously it's something I need to keep an eye on though.

Edited October 14, 2019 by msacco