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Keeping 12V starter battery topped off with 24V RV battery

taiyoken

taiyoken

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Joined
Sep 3, 2022
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Hi there, I’m wondering how I can keep my 12v flooded starter battery charged. I have a 24v lithium system for my camper van, and I’d like to top off the starter when I have capacity (or am charging, such as via shore power or solar). My first thought was to use a Victron Orion 24/12 charger (I use a 12/24 to charge my house battery via my alternator), but I’m imagining there has to be a cheaper way. I’ve seen small MPPT solar panel maintainers for around $80 and I’ve used my AC battery maintainer (which was around that much), so something about dropping $200 on the Victron makes me think I might be overthinking it. Does anyone have a recommendation for how to approach this problem?

On another note… if I were to splurge for the Victron, I’d love to power my 12V appliances as well (I currently have a cheap UXCELL converter), but in my mind it’s one or the other— charging the starter or powering my 12v appliances. That said, am I overthinking that as well?
 
taiyoken said:
Hi there, I’m wondering how I can keep my 12v flooded starter battery charged. I have a 24v lithium system for my camper van, and I’d like to top off the starter when I have capacity (or am charging, such as via shore power or solar). My first thought was to use a Victron Orion 24/12 charger (I use a 12/24 to charge my house battery via my alternator), but I’m imagining there has to be a cheaper way. I’ve seen small MPPT solar panel maintainers for around $80 and I’ve used my AC battery maintainer (which was around that much), so something about dropping $200 on the Victron makes me think I might be overthinking it. Does anyone have a recommendation for how to approach this problem?

On another note… if I were to splurge for the Victron, I’d love to power my 12V appliances as well (I currently have a cheap UXCELL converter), but in my mind it’s one or the other— charging the starter or powering my 12v appliances. That said, am I overthinking that as well?
Click to expand...
A small 12V pv system or a $10-30 buck converter ran manually with a switch or timer switch.
 
Any recommendations for a small charge controller and small panel? I’ve seen lots of 10W-ish panels with built in controllers on Amazon (like this one), but I worry about the quality/reliability and imagine the plastic on the panel would warp over time.

One thing I like about the panel/controller linked above is that it specifically mentions it can stay connected to the vehicle while you’re driving. I don’t want to have to worry about disconnecting it whenever I go somewhere.
 
Ha, that’s genius! I really don’t see why that wouldn’t work for me, and it would be a whole lot simpler (and cheaper)… that might just be what I do ?
 
Rednecktek said:
Umm... what I've done is use a cheap PWM controller. Connect the battery leads to your 12v and the solar leads to the 24v. The controller will keep the battery topped up and the controller just thinks the sun never sets.
Click to expand...
Not a good idea…
The pwm will try to SHUNT the 24V input down to 12v charging voltage…

Better to use a inexpensive MPPT controller.
There are some nice waterproof simple 12V mppt controllers that would work great.
 
Supervstech said:
Not a good idea…
The pwm will try to SHUNT the 24V input down to 12v charging voltage…

Better to use a inexpensive MPPT controller.
There are some nice waterproof simple 12V mppt controllers that would work great.
Click to expand...
An inexpensive MPPT controller for solar, or for connecting battery-to-battery as Rednecktek suggested?
 
And just remember if you put a load on the 12 volt side, it will drain the 24 volt without warning
 
Cool— definitely affordable! And if I change my mind and decide to go PV instead of battery-to-battery, hey, I’ll already have a controller. I like the idea of going battery-to-battery because I already do have a charge controller on the RV battery side (it’s a Growatt all-in-one), but haven’t built out the solar system (yet), and I can use my limited roof space all for one purpose.

Out of curiosity, what is it about MPPT that makes it more effective for this use case? When you say PWM will try to shunt 24v to 12v, how so? I’m familiar with a shunt for the purpose of monitoring current (I have one for monitoring my battery), but what does that mean in this case? Is it that a PWM controller will attempt to direct that extra juice elsewhere (maybe as heat?)?
 
taiyoken said:
Out of curiosity, what is it about MPPT that makes it more effective for this use case? When you say PWM will try to shunt 24v to 12v, how so? I’m familiar with a shunt for the purpose of monitoring current (I have one for monitoring my battery), but what does that mean in this case? Is it that a PWM controller will attempt to direct that extra juice elsewhere (maybe as heat?)?
Click to expand...
The biggest difference in simple terms is that a PWM is like a big resistor and an MPPT is a self-adjusting transformer. Let's do some napkin math* for simplicity:

100w panel putting out 20v and 5a is going to be our example panel and I'm going to say the battery is actually 12v.

100w panel to a PWM takes that 20v and 5a and resists/shunts the excess voltage as waste heat so what comes out is 12v at 5a or 60w. That's 40w of power dumped down the drain rather than into your battery.

Now lets take that 100w panel and CONVERT that 20v and 5a to 12v at 8.3a through diode and transistor magic. Now you're getting almost half again more power to the battery.

Now, these numbers are used for example and concept understanding, nothing is that perfect but hopefully you get the idea.

The advantage to PWM controllers is that they are cheap, simple, and "just work" as they don't need to do a lot to convert solar DC into battery DC. They leave power on the table, but for most basic 100w panel/12v systems the difference in real world performance up to about the 300-400w array range is pretty negligible. The biggest disadvantage is the voltage loss. If you were to throw an old 300w house panel on there that ran 30v and 10a it would cut that extra 20v worth of voltage and you'd only get 120w (12v at 10a) out of that panel. That would be PAINFUL!

The advantage of MPPT is that they can be a LOT more efficient on converting the amperage up, they can take higher voltage inputs which makes thinner wire or longer wire runs much easier to deal with, but usually you're going to have to spend $100 or more. The post earlier still blows my mind!

Does that help make sense?
 
Last edited:
Rednecktek said:
The biggest difference in simple terms is that a PWM is like a big resistor and an MPPT is a self-adjusting transformer. Let's do some napkin math* for simplicity:

100w panel putting out 20v and 5a is going to be our example panel and I'm going to say the battery is actually 12v.

100w panel to a PWM takes that 20v and 5a and resists/shunts the excess voltage as waste heat so what comes out is 12v at 5a or 60w. That's 40w of power dumped down the drain rather than into your battery.

Now lets take that 100w panel and CONVERT that 20v and 5a to 12v at 8.3a through diode and transistor magic. Now you're getting almost half again more power to the battery.

Now, these numbers are used for example and concept understanding, nothing is that perfect but hopefully you get the idea.

The advantage to PWM controllers is that they are cheap, simple, and "just work" as they don't need to do a lot to convert solar DC into battery DC. They leave power on the table, but for most basic 100w panel/12v systems the difference in real world performance up to about the 300-400w array range is pretty negligible. The biggest disadvantage is the voltage loss. If you were to throw an old 300w house panel on there that ran 30v and 10a it would cut that extra 20v worth of voltage and you'd only get 120w (12v at 10a) out of that panel. That would be PAINFUL!

The advantage of MPPT is that they can be a LOT more efficient on converting the amperage up, they can take higher voltage inputs which makes thinner wire or longer wire runs much easier to deal with, but usually you're going to have to spend $100 or more. The post earlier still blows my mind!

Does that help make sense?
Click to expand...
That makes a lot of sense! To vastly oversimplify, it sounds very much like the reason I use a buck converter to run my 12v appliances off of my 24v battery, rather than just sticking a big ol’ resistor in the middle and calling it a day. Since power = V*A, dumping the extra voltage as heat (rather than converting it through transistor/diode witchcraft) wastes a ton of power.
 
I replaced the factory 10watt solar panel they installed to 'maintain' the house battery - what a joke - with a 100w panel and a cheapo pwm SCC to maintain the chassis/starting battery. It's always fully charged.

100W panels are about a hundred bucks, and a decent pwm SCC about $35, and some wire and a breaker.

This is the only renogy item I have and it's been fine. A 10A 'waterproof' solar controller. LOL



If I were to do it again I would use a Trikl-start or similar, but they need a 12V source.
 
An MPPT can not be used as it will try to lower the impedance of the input to try to determine max power point. You will fry the input of the MPPT. There is a reason an MPPT as a short circuit current spec for the PV array that is attached to it.

A much better solution is a DC to DC converter like a Victron Orion.

EDIT: I removed my comment about a PWM working. It won't work properly...too much current.
 
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Yes, using a SCC Mppt can be a 12v secondary battery reserved or I even hook up a 480 watt inverter to the load output on the 12 volt side for emergency 120v power. My SCC haves a 40amp load output and it’s programmable
 
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