Poor man's Aux battery charger

[joshik]

I'm trying to come up with a "affordable" way to charge my 18ah SLA battery box off the cig lighter. I'm trying to avoid buying a redarc, renogy, victron, etc dc-to-dc charger for like $300+.

I've been reading the issues with charging through the cig lighter. Mainly, the 13-14volts produced while the engine is on is not enough to properly charge a 12v battery while using a pwm controller and that the battery could draw more power through the cig lighter than the cig lighter or fuse can handle.

Would something like this work:
cig lighter -> 12v to 19v/3a booster (should be within the safe limits of the cig lighter including the fuse) -> CC and CV regulator (is it needed?) -> the 3stage pwm (maybe mppt later) controller -> 18ah SLA battery.
fuses inline where needed of course.

Am I crazy?

 

[DaveInDenver]

You're not crazy. Is the battery a gel (like for a UPS or something) or a maintenance-free SLA/AGM type?

If it's not a gel type your vehicle already has a charging system that you could almost just plug it into the cigarette lighter but you'd want to current limit, both to safely charge and to prevent melting the outlet. Strictly speaking a resistor and a fuse would be all you'd need.

For a gel you'd need to buck the voltage down, they can't tolerate the relatively high voltage of flooded/AGM.

So in either case one of those CV/CC Amazon Chinese buck/boost regulators might be fine, in theory. They have a habit of not being exactly high quality and you won't get a multi-step profile. But for 18 A-hr you don't need much either so CV/CC is acceptable and will probably get you mostly back in a couple of hours driving.

If you want something less expensive than the full-on dual battery systems but still task specific Optimate makes a small DC-DC charger that is suited for things like this. It's used to keep things like wheelchairs charged when carried in mobility vans.

OptiMATE DC to DC 12v 2 Amp Lead Acid & Lithium LiFePO4 Charger Maintainer

This item does not plug into AC Power. Item is powered by 12 VDC battery or 12 VDC Power-Supply.

www.batterystuff.com

 

[TavisB]

I'm using one of these wired to a cig lighter plug to charge an AGM battery when on the move.

https://www.amazon.com/dp/B004R307S6/ref=cm_sw_r_cp_apa_fabc_8RBWM6FRYD9GK6EK8YPS?_encoding=UTF8&psc=1

Sent from my Pixel 3 using Tapatalk

 

[DaveInDenver]

I'm using one of these wired to a cig lighter plug to charge an AGM battery when on the move.

https://www.amazon.com/dp/B004R307S6/ref=cm_sw_r_cp_apa_fabc_8RBWM6FRYD9GK6EK8YPS?_encoding=UTF8&psc=1

Sent from my Pixel 3 using Tapatalk

This seems like a good find (they have 'em on eTrailer, which increases confidence I'd think). Haven't found any spec for it so I'd hesitate using with a gel type without checking first. But probably work great for other lead acid types.

 

[joshik]

You're not crazy. Is the battery a gel (like for a UPS or something) or a maintenance-free SLA/AGM type?

If it's not a gel type your vehicle already has a charging system that you could almost just plug it into the cigarette lighter but you'd want to current limit, both to safely charge and to prevent melting the outlet. Strictly speaking a resistor and a fuse would be all you'd need.

For a gel you'd need to buck the voltage down, they can't tolerate the relatively high voltage of flooded/AGM.

So in either case one of those CV/CC Amazon Chinese buck/boost regulators might be fine, in theory. They have a habit of not being exactly high quality and you won't get a multi-step profile. But for 18 A-hr you don't need much either so CV/CC is acceptable and will probably get you mostly back in a couple of hours driving.

If you want something less expensive than the full-on dual battery systems but still task specific Optimate makes a small DC-DC charger that is suited for things like this. It's used to keep things like wheelchairs charged when carried in mobility vans.

OptiMATE DC to DC 12v 2 Amp Lead Acid & Lithium LiFePO4 Charger Maintainer

This item does not plug into AC Power. Item is powered by 12 VDC battery or 12 VDC Power-Supply.

www.batterystuff.com

its a Sealed Lead Acid for now. I might go lithium later.

I just need to provide CC and CV to the battery so the pwm controller can run through the stages for charging... but as you mentioned, i'm sure these cheap pwm controllers are not that accurate but probably good enough for an 18ah setup.

I will check out the Optimate. I knew there has to be more options out there than your typical expensive Redarcs and Cteks. Thx!!!

I'm using one of these wired to a cig lighter plug to charge an AGM battery when on the move.

https://www.amazon.com/dp/B004R307S6/ref=cm_sw_r_cp_apa_fabc_8RBWM6FRYD9GK6EK8YPS?_encoding=UTF8&psc=1

Sent from my Pixel 3 using Tapatalk

wow, I will have to check this out!!!
***update***
i looked at this and can it be that simple? This is exactly what I was looking for. I'm not looking for a charging system that can charge the battery quickly. I just need a trickle charge to keep it topped off or top it off after usage at night for phones, cameras and some LED lights.

So this connected to my cig lighter in the back of my 4runner and then connected straight to the SLA battery box (skip the solar pwm charge controller) would work just fine?

 

[DaveInDenver]

its a Sealed Lead Acid for now. I might go lithium later.

So this connected to my cig lighter in the back of my 4runner and then connected straight to the SLA battery box (skip the solar pwm charge controller) would work just fine?

Leaving the solar controller in the circuit would solve any chemistry question since presumably it would matched. You can feed a solar controller with a power supply (e.g. a 120V converter, alternator, etc) but it's somewhat redundant unless you need it to buck down for gel and doesn't let you eliminate a current limit. Leaving it won't hurt anything since it'll probably just go to 100% duty cycle and track the alternator anyway. You just need to make sure you limit current regardless.

 

[joshik]

Leaving the solar controller in the circuit would solve any chemistry question since presumably it would matched. You can feed a solar controller with a power supply (e.g. a 120V converter, alternator, etc) but it's somewhat redundant unless you need it to buck down for gel and doesn't let you eliminate a current limit. Leaving it won't hurt anything since it'll probably just go to 100% duty cycle and track the alternator anyway. You just need to make sure you limit current regardless.

Cool. Thanks for the info! I bought the Optimate, the Tekonsha and this:
Aweking Waterproof DC/DC 12V Step Up to 19V 5A 95W Voltage Boost Converter Regulator Transformer Power Supply for Car Truck Vehicle CE listed

I'm going to try out all 3.

Scenario 1:
Cig lighter -> Optimate -> battery

Scenario 2:
Cig lighter ->Tekonsha -> battery

Secenario 3:
Cig lighter -> booster (output of 19v @ 5amps) -> pwm controller
This one I'm calculating that the input would be 12-14v @ 6-8amps which should be under the cig lighters capacity.
This booster, it would put out a constant 19v at 5amps, right? The amp draw into it would vary depending on how many volts input it is receiving (fluctuations from the battery/alternator)? Am I understanding these concepts correctly?

I like scenario 3 since it would charge the little battery pack faster. Scenario 3 seem sound/safe?

 

[hour]

I don't know why but something has lead me to believe there can be issues supplying a PWM charge controller from a source like you're talking about. I was going to do my typical power supply -> charge controller setup on a renogy PWM (wanderer, if it aids in your googling) and found a thread stating this is only OK on MPPTs. I then googled some more and found talk of it beyond Renogy products.

 

[joshik]

I don't know why but something has lead me to believe there can be issues supplying a PWM charge controller from a source like you're talking about. I was going to do my typical power supply -> charge controller setup on a renogy PWM (wanderer, if it aids in your googling) and found a thread stating this is only OK on MPPTs. I then googled some more and found talk of it beyond Renogy products.

hm, im not sure why it would be any different? As long as it's DC and within a certain voltage/amp range for the PWM controller.

Right now, I am running this pwm controller (ALLPOWERS 20A Solar Charger Controller Solar Panel Battery Intelligent Regulator with USB Port Display 12V/24V). I am connecting a 18w/18v/1amp solar panel to the controller. I believe this can handle up to 20amps of input current so running 19v at 5amps should be ok? It'd be like I'm just running a stronger solar panel? There is no inversion or anything like if I used a dc-ac inverter then used a ac-dc wall brick converter. I think that is where sine wave issues come up? I'm not an electrical engineer (I am a network engineer so I tend to pickup on diagrams/logical stuff quickly), just picking things up as I read/research.

...but changing to a MPPT controller wouldn't be the end of the world. It's better anyways and I was thinking of doing it down the line...

 

[DaveInDenver]

Both PWM and MPPT solar controllers may or may not work with a power supply feeding them. Success stems from the assumption that the solar panel is current limited and that eventually a rise or fall in current has relationship to voltage fall or rise so the controller know it has found the maximum power. In all cases the power supply, controller and battery have to be compatible insofar as you don't test absolute maximums (e.g. current limiting, don't grossly oversize the power supply) and that the power supply knows its current limit and doesn't let the controller step past it into fault.

 

[OllieChristopher]

Best bang for the buck for cig charger is a small and cheap Solar panel charger like the POWOXI 3.3W-Solar-Battery-Trickle-Charger-Maintainer -12V Portable Waterproof Solar Panel Trickle Charging Kit. You can get these off amazon for like 30 bucks. Just shove the plug into you lighter and forget about it. Your battery will always stay at a safe voltage. Used one for almost a year with zero issues and it always stayed plugged in when the truck sat for months at a time.

 

[joshik]

Both PWM and MPPT solar controllers may or may not work with a power supply feeding them. Success stems from the assumption that the solar panel is current limited and that eventually a rise or fall in current has relationship to voltage fall or rise so the controller know it has found the maximum power. In all cases the power supply, controller and battery have to be compatible insofar as you don't test absolute maximums (e.g. current limiting, don't grossly oversize the power supply) and that the power supply knows its current limit and doesn't let the controller step past it into fault.

ah icic. Wouldn't a step up be current limited... in the case of the one linked.... 19v at 5amps? Would the pwm controller not think its a 95w solar panel?

 

[joshik]

Best bang for the buck for cig charger is a small and cheap Solar panel charger like the POWOXI 3.3W-Solar-Battery-Trickle-Charger-Maintainer -12V Portable Waterproof Solar Panel Trickle Charging Kit. You can get these off amazon for like 30 bucks. Just shove the plug into you lighter and forget about it. Your battery will always stay at a safe voltage. Used one for almost a year with zero issues and it always stayed plugged in when the truck sat for months at a time.

yeh or the ones that dave and tavis recommended would work like that. I was just trying to eek out a little more power for a little faster charging...

 

[jonyjoe101]

for something like what you want to do, you need a boost buck converter. Thats how I charge from 12 volt to 12 volt. You need the boost converter to increase the 12 or 14 volt to 18 volt, then you need a buck converter to drop the 18 volt to 14.4 volt or whatever you need. Using the boost/buck together you will get the max amps possible.
If you try to just use a boost or buck by itself, you won't get max amps, the input/output voltages will be too close together to get max amps. Just make sure you use a cooling fan, the converters get hot.
The boost and bucks each cost about 8 dollars, and I also use a volt/amp meter (5 dollars) to monitor voltage/amps. This is a cc/cv charger so it will give your lead acid a good charge, I also use to charge my 12.6 li-ion an 14.6 lifepo4. This one charges at 6 amps. The output volt/amps are adjustable. With the volt/amp meter you can tell at realtime when the battery is almost full, when the charge amps drop to about 100ma.
It looks complicated but its easy to build and works great.


note the volt/amp meter in front

 

[DaveInDenver]

ah icic. Wouldn't a step up be current limited... in the case of the one linked.... 19v at 5amps? Would the pwm controller not think its a 95w solar panel?

A power supply doesn't have the same characteristics as a solar panel.

These are just hasty example ideal sources, both being theoretical 85W sources. The power supply in this case would be rated 14V@6A. The solar panel would be Isc=6.5A, Voc=17.1V and MPP=13.91V@6.2A.



Notice how a power supply reacts, it's always got 85W (or 14V and 6A) available until it doesn't. If you ask it for more than 6A the voltage drops and you can't ever get more than 14V but is constant under 6A. The PV power will be a function of voltage and current and neither of them are fully constant (current is practically up to MPP at least). In particular notice how the PV power forms a distinct peak that falls off both sides.

In the design of power supplies and solar controllers assumptions are made about the nature of their upstream input sources and downstream loads. In this case substituting a power supply for a solar panel into a controller might demonstrate where assumptions made are not capable. Sometimes it works just fine, too.

It's usually MPPT where you'll find the limits of doing this. If you assume there's not a cliff to walk off, which is a safe assumption with a solar panel, you don't need nearly as robust of an MPPT design. Better MPPT controllers will measure current in or out to know they've hit set points or limits. Those could run happily at their limit as long as the power is stable.

Others will walk to the edge and go right over to find the fold back current of the supply or exceed their own power limits.

PWM can work if the power supply and battery are compatible but will run into issues if you ask the PWM to match them beyond it's limits, which means a max voltage difference and current limit.