Portable starter battery maintainer build

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The goal is to keep the battery in my infrequently used car well charged.
I park on a city street with no access to AC power.
I used an inexpensive solar panel (x2) on the dashboard for a couple years; its performance tapered off to failure.
Bought another, same degradation and failure ... in 2 months.
Before that I'd take the battery out ~4X per year and charge it overnight inside.
Time for something new.

For safety and durability I start with some LiFePO4 cells, 4s2p, left over from an e-trike experiment. (These were a lot cheaper 4 years ago).
Active balancer for easy charging on a bench supply.
Feed the ~13V battery output to an adjustable boost converter set for 14.xV output, 6A current limit, 12.5 low V cutout.
Monitor output with a small V & A display.
Attach the output to the car with an OBD power cable, fem connector snipped off & replaced with quick connect.
Replace the fuse in the cable with 7.5A (ATM size) to avoid ever blowing the car's 10A fuse to the comm port.

This gadget will sit in the driver's footwell and charge the car's battery autonomously every few months.
There should be enough capacity in the LiFePO4 battery (20AH) to supply 1-2A overnight.
There's the low voltage cutout to stop it otherwise.
Pictures when I get some more parts in and more assembly.
 
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So you're using a LiFePO4 battery to maintain a battery overnight? Other than the possibility of burning down your car, isn't the point of a maintainer to feed the battery over long periods?
 

circuitsmith

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I've assembled UV cameras to go in space.
I think I can manage to construct a fairly simple automotive accessory without combustion.
LiFePO4 are relatively fire resistant.

Re calling it a maintainer: periodic full charges are a helpful part of lead-acid battery maintenance.
Not to be confused with a "battery tender" that stays connected for days, weeks, months.
 
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Chevy Bolts and even Teslas burst into flames for no apparent reason and I'm sure they are designed by top notch engineers. Just saying that cobbling together something from an ebike battery is asking for trouble. You say you tried inexpensive solar maintainers specifically designed for this? Maybe spend a little more and get higher quality. I've used the VW chargers that plug into the obd2 socket and they keep a battery topped up for years.
 

circuitsmith

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The solar panels had downsides, even when they worked:
Taking the panel off the dash when I drove, or risk a potential projectile, plus reflections on the windshield.
Jockeying for a parking space not under a tree or in shadow etc.
Less sun in the winter.
 

circuitsmith

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Batteries, booster board, OBD cable, carry bag.
P1020216.jpg
P1020214.jpg
 
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Timing circuit would be neat… wake up periodically, auto-check voltage, lift it back up, maybe not to 14 but maybe 12.8 or 13? Shut down. Glow a high-bright LED when it’s internal charge is low?

really neat project.

I’ve got PV on the camper shell of the truck and it’s Great. Great enough that I’ve considered a surface mount cel on the trunk lid of the sedan. But that’s drilling holes… and it’s a pretty car; cant quite do it.

i understand your goals…
 

circuitsmith

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Timing circuit would be neat… wake up periodically
I could see that if it were used long term.
But it will be hooked up just for a few hours every couple months.
And parked on a city street, I don't want any bright lights to attract attention.

For a long term setup I would put it in the trunk, use relays to switch to charge the LiFePO4 bat when the car is on, and float the car batt at 13V when the car is off.
 
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Exactly which solar battery chargers were you using that didn't last?

I'd suggest buying a better quality solar maintainer or building your own with better quality materials.
 
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I would drive it every two to three weeks and forget trying to use another battery to keep it charged.

Using the car will be better for the car and discourage mice.

Let a friend borrow it.

If you really use it infrequently sell by it and consider renting.
 
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I am trying to get my head around this modern battery types - circuits and charging I am good with.

So your batteries are rated at 10,000mAh -
  • Nominal: 10000mAh (3.8V cut-off, 1C )
How much real world 12V mAh do you think you would get out of such a battery. Thinking about making my own power station, or possibly a charge battery to add to the power station - similar to your battery maintainer setup.
 

circuitsmith

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I'd suggest buying a better quality solar maintainer or building your own with better quality materials.
See my previous post about why I'm no longer interested in a solar panel.
I would drive it every two to three weeks
I already drive 1-2 times each week.
Using the car will be better for the car and discourage mice.
I've already had a run-in with rodents.
I'm not willing to drive all night to keep them away.
Let a friend borrow it.
The way people drive around here? Are you nuts!?!?
The less it's on the road locally the safer I feel.
So your batteries are rated at 10,000mAh -
  • Nominal: 10000mAh (3.8V cut-off, 1C )
How much real world 12V mAh do you think you would get out of such a battery.
The battery is configured for ~13V and 20,000mAH, ~260WH.
For LiFePO4 I have 3.1V per cell cut-off.
I'm looking for capacity to do an overnight 8-10 hour charge @14.2V and 1.5A average , which is ~220WH.

I did do a 4 hour test run after the car sat for a week.
The charging current tapered off to ~800mA (after making sure the car was "asleep") after the first 3 hours, delivering total 62WH.
 
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Awesome project.

Years ago , i got a 150 watt voltage booster, slow boat, for less than 2$ delivered.

I installed it inside some ancient ventilated inline power brick and put a 50mm fan across its heatsinks, and 45 amp anderson powerpole connectors as input and outputs, also a voltmeter.

Modified the voltage potentiometer with a 10 turn bourns for easy, precise adjustment of output voltage.

I typically use my rc style inline ammeters/ wattmeters on the output.

Ive got some portable 12v agms which want 14.5 to 14.9v absorption, other gc-2 dekas that want 14.1 to 14.4v absorption.
Solar brings the dekas tobor toward 14.1v, while my inline voltage booster holds the 'portable' agms at 14.8v, until amps taper to 0.1a, or less.

With flooded bstteries my inline v booster can perform battery to battrty desulfating 16.2v equalization charges.

Some People cant understand just how long it will takes to get a lead acid battery, from even 95% to 100%..

it Can easily take Hours, when held at high voltages, for amps to taper to their minimum, or for specific gravity to max out, before overcharging begins.

True 100% recharges, applied regularly, is the best way to insure very good battery longevity.

I once used SAE 12v connectors. No more.
Anderson powerpoles are so much better.
No worries about source becoming load, and reversed polarity magic smoke escaping.

I stuff 8awg into the 45 amp anderson contacts .
A parallel pair have 'barely' started my V8, from a new 35$ 11lb ub12180 battery.

Ive also started a 8 month cold SB chevy with a ub12180 and ub12220 in parallel, through a pair of 45 amp andersons.
Try that with sae connectors on 10awg, and watch them melt.
 
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If you drive it 1 or 2 times a week and are not getting the battery fully charged then you are probably not driving very far and that is an issue by itself.

New cars don't use a lot of power to start the car.

An AGM battery would self discharge less.

Have you measured the drain on the battery while the car is just sitting.
 
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If you drive it 1 or 2 times a week and are not getting the battery fully charged then you are probably not driving very far and that is an issue by itself.

New cars don't use a lot of power to start the car.

An AGM battery would self discharge less.

Have you measured the drain on the battery while the car is just sitting.
I am nowhere near as knowledgeable as the OP but it depends on the vehicle and battery.

As mentioned, getting a lead acid battery to full charge takes a long time, and todays alternators / smart charging systems usually taper down or turn off to save fuel long before you get there.

AGM's are better but also won't get to full charge with the amount of voltage most alternators put out. If you want to maintain an AGM to full potential you need to put it on a AGM specific charger once a month or so to "top it up" so to speak.

If you just use your alternator to charge, your likely only getting the battery to maybe 90% of its potential - which is completely fine for most people, but some do this sort of thing mainly as a hobby - just like long OCI's and thinking more than 2 seconds about what kind of oil to buy.
 
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P
I am nowhere near as knowledgeable as the OP but it depends on the vehicle and battery.

As mentioned, getting a lead acid battery to full charge takes a long time, and todays alternators / smart charging systems usually taper down or turn off to save fuel long before you get there.

AGM's are better but also won't get to full charge with the amount of voltage most alternators put out. If you want to maintain an AGM to full potential you need to put it on a AGM specific charger once a month or so to "top it up" so to speak.

If you just use your alternator to charge, your likely only getting the battery to maybe 90% of its potential - which is completely fine for most people, but some do this sort of thing mainly as a hobby - just like long OCI's and thinking more than 2 seconds about what kind of oil to buy.

The battery chargers I have that have an AGM option say to only use that option for the high end AGM (spiral wound, etc). Not your $150 AGM from Walmart. Of which I have two in my F250.
 

circuitsmith

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I once used SAE 12v connectors. No more.
Anderson powerpoles are so much better.
I keep a bag of PowerPoles handy, and have used them extensively since my first e-bike project 20+ years ago.
Amusingly, this one time I felt sorry for a bag of hand-me-down 14AWG SAE connectors, headed for Goodwill, become the star of the show.
 
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