Charging smaller battery from main setup

[hour]

Kind of like the million b2b questions but I figure with the solar generator craze going on right now (see: Amazon), with most/all having car charge adapters, that there might be some news on how to most easily accomplish what I'm trying to do.

The primary thing I run with my setup is my whynter dual zone fridge. With 200w of solar I find myself in float mode until the sun goes down, so I have a surplus of power. The battery setup is already like 135lbs so adding more storage isn't desired, nor beneficial for the task... I just want to make use of this surplus energy.

Main setup on top of / in back of truck:
2x100w panels in series to a Victron 75/15 solar charge controller
2xDuracell 6v golf cart batteries in series for 12v.

What I want to use this surplus energy to charge/operate:
A large DIY stereo I built in to a toolbox with an 18ah lead acid battery inside.

I want to sit this toolbox on the back of my tailgate plugged in to my house batteries so that it can play and charge during the day. Then when the sun goes down and I no longer have a surplus of power, I can unplug the toolbox and move it closer to the campfire, and let the house batteries just run the fridge.

This could be accomplished with pretty much any of the solar generators on Amazon since their inlets accept something like 13-30v / 40w max (solar/car dc inlet) and my batteries will be 13.5-14+v when excess solar is being produced, but it's another thing to buy, and a pre-made solution won't fit inside my toolbox unless I gut the unit. I already have a separate charging system for the toolbox boombox - dedicated solar panel and controller, but it's a pain in the ass and I'd rather just leave the extra panel at home and keep things on the tailgate during the day.

Ideas appreciated

 

[dwh]

Just plug it into the house bank. The charge voltage for the house bank will work fine for an AGM (assuming the small battery is AGM).

If you want to get fancy...

https://www.solar-electric.com/xantrex-82-0123-01-echo-charge.html

 

[hour]

Just plug it into the house bank. The charge voltage for the house bank will work fine for an AGM (assuming the small battery is AGM).

If you want to get fancy...

https://www.solar-electric.com/xantrex-82-0123-01-echo-charge.html

The 6v house batteries are not AGM, nor is the small battery, and reason for not wanting to directly connect them is that the boombox might see a lot of play after-hours and wind up extremely far off from the house batteries voltage come morning. I hadn't thought about treating this connection in the same manner as an isolated dual battery setup, so that's something I'll definitely consider pricing out-thanks.

The other thing I'm loosely toying with is utilizing the toolbox boombox's internal solar charge controller and a "8-16v in -> 16.3v out 6 amp 98w" step up module. That'd provide enough voltage to cover all phases of battery charging for the little battery, and sustain the system during daytime playing (20-30watt draw) while also charging it up. Certainly not the most efficient route, but a cheap way to utilize the surplus power.

Thinking with that option, I could just use the load output off the house charge controller (programmable via app, say on at 13.3v off at 12.8v) to supply the step up module with power, and the step up module feeding the toolbox's solar charge controller via solar panel input.

 

[dwh]

The 6v house batteries are not AGM, nor is the small battery, and reason for not wanting to directly connect them is that the boombox might see a lot of play after-hours and wind up extremely far off from the house batteries voltage come morning.

The voltage difference won't matter. The voltage potential coming out of the Victron will be higher than either the house battery or the boombox battery. Power will flow from the Victron to each battery seperately depending on each battery's ability to accept current.

And if the batteries are both lead-acid, then the voltage that works for one will work for the other.

You don't actually need anything special to charge them together.

 

[hour]

Maybe I've been reading and youtubing too much about lithium systems. Something something current inrush.

Any thoughts on utilizing the switched load output (rated 10 amps) for some sort of isolation effect, like forget on tailgate all night playing, plugged in, and after the house battery drops below float voltage, the load output shuts off and the boombox will just use internal power? I'm unclear on what would happen if an 18ah battery sitting at 12.3v from the night before gets directly connected to a fused output potentially sitting above 14v.

That aside, it's effectively paralleling a large system sitting at a maximum state of charge, to a near dead system less than a 10th the size. And that's ok?

Sorry to sound like a skeptic. Just always thought there was a lot more involved than that.

 

[dwh]

If you want the technicalities...

http://www.smartgauge.co.uk/nosurge2.html


In a nutshell, with lead-acid to lead-acid, there is no big current surge. Both batteries are giant resistors, and the resistance mitigates against big surges.

A lithium could suck up more power faster than a lead-acid, but lead-acid to lithium would still be mostly mitigated by the resistance of the lead-acid.

Lithium to lithium you might have to keep an eye on it.


As for using the load outputs on the Victron...sure.* It's going to be exactly the same as plugging direct to house battery, but with the LVD (low-voltage disconnect) added. I assume the LVD can be programmed via the Victron app? My Victron 100|30 doesn't have load terminals.


As for what happens when you plug a 12.3v battery into a source with a voltage potential of 14v+...

Nothing. The battery regulates the voltage of its own unique charging circuit. Say the house bank at 12.6v and the boombox at 12.3v. The charging loop (circuit) of the house bank will be operating at 12.6v and the loop for the boombox at 12.3v. The voltage of either circuit won't rise to 14v+ until the battery allows it to rise - but that doesn't happen instantly.

You can create an entirely new circuit by putting a meter across the terminals of the charge controller, and that circuit will be operating at the full potential voltage of the source (minus a teeny-tiny bit due to the resistance of the meter).

Each battery will regulate the voltage and current on its own charging loop, until the voltage (or current) rises high enough for the charge controller to take over imposing limits.

 

[dwh]

*Um...provided of course that the boombox+battery does not exceed the amp rating of the load terminals, which is probably 10a.

 

[hour]

*Um...provided of course that the boombox+battery does not exceed the amp rating of the load terminals, which is probably 10a.

Uh oh edit: I just learned a lot of these black UPS-type batteries that I use are 'AGM Sealed Lead Acid' batteries and the truck bed batteries flooded lead acid. Implications?

Great, the post above explained a lot. There are some DIY concepts that I've never been able to grasp and I've done more than a few projects only to look back years later and wonder how I concluded what I did was most appropriate. I research the things I try to assemble obsessively.

Example in this instance: I've built multiple < 36ah 12v SLA camp packs (toolboxes, ammo cans) for lighting outposts & tent friends with air mattresses and gizmos, in large middle-of-nowhere camp sites. It's no big deal to round them up and charge them at the truck in the morning, but I've always assumed I either needed whatever safety/boosting/regulating gadget exists inside a Goal Zero Yeti / ArkPak that allows it to charge DC-DC... or, as I've done a couple times, use an inverter and battery charger powered from truck setup. The inverter setup obviously isn't ideal, I don't want to purchase nor carry a large and powerful battery charger (not sure what max in current is even acceptable for say 2x9ah UB1290's in parallel), and having a smaller 2ah battery charger running the entire day off an inverter is no good either.

So because of this lack of understanding, thinking I couldn't parallel the small batteries to the truck setup, I have:

• A bunch of extra solar panels that suck to transport, hang on bushes, prop up, pick up after a wind rolls through
• A bunch of SAE/MC/Anderson Powerpole gizmo patch cords I've assembled to standardize wiring. And all of the proper $ crimping tools.
• A whole milk crate full of tangled uncooperative cords
• A plethora of charge controllers, some encased in external waterproof boxes
• And have made design sacrifices to accommodate charge controllers internally for some projects

When you say provided the boombox+battery does not exceed the amp rating of the load terminals, say it's connected and boombox part turned off. It will draw current from the load terminals. I'm assuming that amount is determined by the overall resistance, and influenced by the fact that I have two 9ah batteries in parallel.

So even if the boombox part is off and I connect it to the load output from main setup, the batteries will pull at some rate, hopefully well below 10 amps. And I'm speculating that their starting voltage, and the supply voltage from load output, could dictate the rate of transfer and demand on fuse. And that assuming for instance (I have zero knowledge of any real life numbers) 5 amps power was being transferred to the battery from the load, and then I cranked the boombox to 100 watts, that plus the 5 amps would be bad news for the load output circuit. It'd be really rather slick to pull this off without any additional items though I guess a relay could help me dodge the 10 amp deal. Trying to figure out now what the max draw of the battery specific would be when paralleled

Thanks again for the education!

 

[hour]

I guess at the end of the day I may as well be connecting a lead acid jump pack, which have existed forever and use the same type of batteries, to a 10 amp fused cigarette lighter in a driving car. So in googling that, I find people still discouraging it, or mentioning that the good portable jump packs have a circuit to regulate current, or others a "big sand resistor". Saw mention of not wanting to trust that current draw would be below 10 amps when at a low state of charge hitting charging vehicle voltages. Also a "Even a "lighter-lighter charger" probably isn't going to be current limited to the point where it's appropriate for a little 7Ah battery. " and my portables are a bit larger than that. Additional mention of portable jump packs and similar being up to 15 amp hours, suggesting that maybe a dead 15 amp hour battery would be the max that, from a low state of charge, be able to charge from a typical cig lighter without blowing anything.

So wondering now if I should do a ~30a relay off the switched 10a load from controller, because that'd put my project cost at a couple of bucks. If I could find dual battery systems scaled down small enough for this, with the price reflected, I'd probably just do that. But this is the only time I've tried to find a dual battery package with less capacity...

 

[jonyjoe101]

As long as all the batteries or about the same voltage (around 12 - 14 volts) it will work, I do it all the time to top off my smaller batteries, just plug it into my house battery. I done it with my 18 ah jump pack, also 28 ah agm, I even plug in my 65 ah 11.1 volt li-ion. Amp current from the house battery 220ah lifepo4 (reading over 14 volts) to the smaller battery is minimal even when the battery is not fully charged, I see maybe 6 amps. Even the li-ion when its reading 11.5 volts, there is no massive rush of amps going into it.

I use the xt60 connectors on all my batteries, I don't get any sparks, cables don't overheat, fuses don't get blowned. There is no way to overcharge/damage a lead acid with 14 volts of power, you need 15 or more volts to damage a lead acid.
I also connect a jump pack (18ah agm) to the cigarette plug on my van to charge, no fuse blown, all jump packs can be charged that way. I use a dc wattmeter (12 dollars) and can measure the amps going from house battery to the smaller batteries, and I never see more then 8 amps going to the smaller battery. The smaller the battery, the less amps it receives.

I use 10/12 gauge wire on all my battery connections, maybe thats why I don't see too many amps. I don't know if larger gauge wire will make a difference.

The small 18ah agm batteries in jump packs are rugged, I use to use one as my house battery connected to my 240 watt solar panel. They won't be damage easily when charging.

 

[dwh]

Uh oh edit: I just learned a lot of these black UPS-type batteries that I use are 'AGM Sealed Lead Acid' batteries and the truck bed batteries flooded lead acid. Implications?

(thought so... /me = psychic)

Implications? None.

For the most part, lead-acid is lead-acid. They all work the same with basically the same charge voltages and/or charge profiles. Oh, charge profiles can be optimized a touch here and there. Flooded and AGM actually do best when bulked to 14.8v, but most multi-stage chargers stick with a nice safe 14.4v that works on anything. (Though some GELs don't like being above 14.2v for any length of time, as the heat tends to break down the gelling agent used in the electrolyte.) Of course, with flooded, bulking them regularly to 14.8v, while being very healthy for them, will consume more water. There are a few outliers, like Optima, which due to the spiral-wound AGM design have a somewhat lower resistance curve and like a bit more voltage, or Odyssey which due to their high-compression thin-plate pure-lead AGM design like a nice heavy amperage flow.

But hooking up a small AGM to a big fat FLA bank and charging them together isn't going to do anything bad. The little AGM will still absorb power at whatever voltage/current its resistance allows until the electrolyte is fully saturated with electrons. Plus, getting a multi-stage (multi-voltage really) off the Victron is going to feed it just as well as it feeds the FLA bank. It won't overcharge since it will live happily at the same voltages as the FLA bank. And no matter that the FLA bank is huge compared to the little AGM - if they are at the same voltage, then there is no difference in potential, and no power flows from one to the other. If they are connected to a source with a higher potential voltage, then the source will supply them both and they won't feed each other.

BUT...the little AGM might just have a low enough internal resistance to occasionally suck up more than 10a, and if the LVD on the Victron is limited to 10a, you might have a problem. But you can jimmy that a bit by simply using smaller wire to feed from the Victron to the boombox. This will increase the resistance and choke off the peak amp flow somewhat. Ultimately, the voltage will still rise to equal the source (voltage drop doesn't really apply when charging batteries), so the thinner wire won't stop the battery from reaching a full charge - it will simply take longer due to having the amp flow throttled.

When you say provided the boombox+battery does not exceed the amp rating of the load terminals, say it's connected and boombox part turned off. It will draw current from the load terminals. I'm assuming that amount is determined by the overall resistance, and influenced by the fact that I have two 9ah batteries in parallel.

Exactly.

And I'm speculating that their starting voltage, and the supply voltage from load output, could dictate the rate of transfer and demand on fuse.

Correction: not "supply voltage"..."supply voltage POTENTIAL". The source, say the Victron, might have a potential of 14.8v or whatever, but the actual working voltage of the charging loop will be at battery voltage. This is the problem with a PWM solar charge controller. Since it is simply a high-speed chatterswitch, when the solar is connected to the battery, the solar is forced to operate at battery voltage, rather than at the solar panel's optimum Vmp. MPPT separates the setup into two loops, one for the solar, and one for the battery, which can operate at different voltages and thus get 100% of available power from the solar.

So the starting voltage and the supply voltage don't really mean much. What matters is the resistance of the battery. Lead-acid battery resistance is plotted on a curve, with the lowest resistance around 50% charged and higher at both ends (low state of charge and high SoC). There is also surface charge to consider, which is where electrons build up at the interface between the plates and the electrolyte. During charging, this causes the voltage of the battery to rise, which increases the resistance and throttles the amp flow rate. Then it takes time for the built-up electrons to leech or "absorb" into the electrolyte, so you just hold the voltage up high and wait however long it takes for the electrolyte to fully saturate.

And that assuming for instance (I have zero knowledge of any real life numbers) 5 amps power was being transferred to the battery from the load, and then I cranked the boombox to 100 watts, that plus the 5 amps would be bad news for the load output circuit.

Maybe. Sound amplifier "watts" is not the same as "electricity watts". A 100w amp might not actually be drawing 100w of power. But yea, you get the idea. Battery will suck X amps, radio/amplifier will suck Y amps. Together they can't exceed Z amps the Victron's load circuit is fused for.

It'd be really rather slick to pull this off without any additional items though I guess a relay could help me dodge the 10 amp deal.

Meh. I'd just plug it into the house bank and let it absorb whatever it wants. Use big enough wire to feed whatever it could possibly draw plus whatever the radio could possibly draw and stick a fuse in it. But if you really just gotta have that LVD, there are plenty on the market that can handle more than the 10a of the Victron's built-in LVD.

https://www.amazon.com/s?k=low+voltage+disconnect+12v+30a&i=industrial&ref=nb_sb_noss

Thanks again for the education!

You can owe me a beer.