Solar with 12V Lead Acid Car battery? I know it's a bad idea but...

Solar with 12V Lead Acid Car battery? I know it's a bad idea but...
Posted by jwhitmore on 03 Mar, 2018 19:36
OK Before I start I have read that it's a really really bad idea because you'd have to constantly monitor the batteries to make sure they were still alive, kicking and healthy. Even if you're not using them as deep cycle batteries but creaming off the top. So given that starting point - "It's a really bad idea" there's no point in responding to this message to say that it's a really bad idea. I KNOW that already. But is there a way around this really really bad idea? Come at it from a different angle?

So imagine that you're in the book "The Martian" and you need electric power. You have access to 12V automotive Lead Acid batteries and unlimited electronics, well all that farnell or digikey can supply, and you need a solution. I know that if you weren't in "The Martian" but instead had just won millions on the lottery the solution would be to buy a new everything, and two of them.

So given that I'm on Mars and I'm trying to "engineer" a feasible solution... I don't have an answer, I just have another question, which might be an equally bad idea... So even if you look into using Li 18650 batteries there seems to be a serious headache of balancing batteries. So there's a problem of monitoring and balancing batteries. Could you overcome these issues with an IC that can monitor a Lead Acid battery? I'm looking at http://www.ti.com/lit/ds/symlink/bq34z110.pdf but that says it's not suitable for new designs. I'm not sure that would even tell you that a battery was bricked and should not be connected to the load, or charged. Then it is quite an expensive IC but at least you're talking about adding that IC to a lead acid battery not an 18650. So first question how to determine the charge level and health of a 12V Lead Acid battery. Is that smaller problem solvable? Is there an alternative to the TI-BQ34Z110? There's lots of fuel gauges for LiPo but Lead acid not so much.

So I'm not sure about the usefulness of the TI-BQ34Z110 but lets assume for a moment that you had information on 10 lead acid batteries. The other thing I'm wondering is could you NOT attempt to balance the batteries (NOT use them in parallel in a 12V System) but use a number of them in series. OK it would take a uC and a lot of switches but if the uC knew the health of the batteries and could control their position in a circuit could the uC put 5 healthy batteries in series giving a nominal 60V. The uC might report that battery 'A' is toast and it's for dumping. But use the remaining batteries intelligently, possibly pulling batteries out of the circuit and replacing them with others to maintain a 60V output for as long as it could.

So each battery 'A', 'B', 'C', ... 'J' Can be switched to any position 1, 2, 3, 4, or 5 in a series circuit by a uC. Given that switching that much power fast is going to be a nightmare it might not even be possible. Still I'd imagine that UPS systems probably intelligently switch batteries it might be possible.

So the uC intelligently switches batteries into "position", in the same way when there's Solar energy to charge a battery the uC intelligently selects a battery to charge and can "switch" the charger to a battery. We know that battery 'A' in this example is 'toast' so don't even bother, but Bulk charge battery 'B'. Once 'B' is up to a certain charge individually do the same for batteries 'C', 'D', 'E'... and when they're all bulk charged connect a constant voltage across all of them in parallel to top them off, (assuming that you've got all 9 batteries up to 12V or something. They're all balanced and can be connected in parallel at this stage, I think.)

If you were going to use an off the shelf inverter, or a 12V off grid DC system the 60V has to be stepped down which is another headache and will introduce losses into the system but... You might need to raid a huge inductor from something which can handle that kind of power. Say for example you were going to power a 1KW Inverter you'd need 1,000/60 = 16.67 Amps. I'm thinking that might require the innards of a welder or something. (I know nothing about power electronics, did you guess?)

Actually that 16.67 Amps sounds a bit high so assuming that any of this is possible you could try for 20 batteries of which you connect 10 together in series to give 120V and a requirement of 8Amps, or there abouts. That 8Amps is better in terms of requiring less heavy duty Mosfets/Switches, but being able to switch 20 batteries to at least 10 positions would be a ridiculous number of switches. It's starting to feel like the BOM used to break the Enigma code during the War. Far far too many switches.

Going to buy a lottery ticket.

#1 Reply
Posted by helius on 03 Mar, 2018 19:46
First of all, a battery is an energy storage device made of several cells in series. The cells are the things that need to be balanced so their state of charge is equal, or they will be damaged by reverse polarity. 18650s are not batteries. Flooded lead acid batteries are monitored by measuring the specific gravity of the electrolyte and deep-cycle units provide ports for that purpose, which are frequently absent on maintenance-free car starter types.

#2 Reply
Posted by Seekonk on 03 Mar, 2018 20:21
That's funny, I live for months each year with just a car battery and a 60V array. Fridge, hot water, dishwasher, the comforts of home. All of the electronics cost less than $50. When I need something, just go out to the garage and whip it up in an hour. How hard is that.

#3 Reply
Posted by vk6zgo on 04 Mar, 2018 09:45
Lead-acid cells have been used for storage for over a century, for instance, 48 volt "battery banks" at telephone exchanges, so there are not a lot of problems.

Of course, they are usually special storage types, designed for long life.
They are usually left in series whilst being charged, rather than disconnected & charged in parallel.
TV & Radio sites often use similar banks to start their standby diesels.

I have even seen such a site which used heavy duty truck batteries appropriately connected to do the same job.

#4 Reply
Posted by ogden on 04 Mar, 2018 10:14
High power UPS systems usually uses lead-acid banks that are connected in series, usually there is no any charge balancing. UPS just monitors all the battery "taps" and report about "weak link" in the chain, if any. Look for > 10kW UPS installation and maintenance manuals, maybe you can even find some service manual with circuits on the web - who knows.

Regarding car batteries - they are just lead acid batteries designed for high currents. This means much bigger electrode area compared to deep cycle battery, this means less "meat" on electrodes. Such indeed can be used for UPS, but lifetime will be lower and you can do absolutely nothing except good (electrolyte) care to prolong it's life. Just derate capacity and maybe do not do deep discharges, adjust "fully discharged" voltage accordingly.

Quote from: jwhitmore on 03 Mar, 2018 19:36
So each battery 'A', 'B', 'C', ... 'J' Can be switched to any position 1, 2, 3, 4, or 5 in a series circuit by a uC.

Don't invent bicycle. Just use bank of 4 batteries so you can use widely available 48VDC telecomm power equipment and that's it. There are ready to use 48 VDC solar controllers, inverters and so on.

#5 Reply
Posted by ogden on 04 Mar, 2018 10:25
Quote from: helius on 03 Mar, 2018 19:46
deep-cycle units provide ports for that purpose, which are frequently absent on maintenance-free car starter types.

Thou, many kinds of them can be "hacked" for maintenance.

#6 Reply
Posted by jwhitmore on 04 Mar, 2018 22:16
Thank you all for your contributions. I didn't get shot out of the water as much as I'd expected ;-)

I should have posted this link [1] It's an old blog post but I'm sure chemistry hasn't changed much. I might get a hydrometer and suck it and see, so to speak. Just do this manually first and see how long it takes to kill a battery. Get some data. Automation comes after manually trying this out and see how it goes. Thanks again.

[1] http://forum.solar-electric.com/discussion/14406/use-of-car-batteries-in-solar-application

#7 Reply
Posted by jwhitmore on 04 Mar, 2018 22:17
That's very interesting can I ask what kind of setup you use for step down from the 60V?

#8 Reply
Posted by DougSpindler on 10 Apr, 2018 13:47
Interesting post. I found there is a design difference between car batteries and storage batteries. Car batteries deliver lots of current for a short duration and will have a long life if kept at 80% of full charge.

Storage batteries are designed for loww current and deep discharges.

Best way to ruin a car battery is to deep discharge it. Sulfur crystals will build up on the lead plates The sulfur crystals are an insulator and will decrease the battereis capacity rapidly. A new car battery used in deep cycling discharges can last less than a year. I have fried who is living out of his car using his battery for deep discharge and he has to ereplace the battery every year or so.

#9 Reply
Posted by Ian.M on 10 Apr, 2018 14:15
s/sulfur/Lead sulfate/

#10 Reply
Posted by Red Squirrel on 12 Apr, 2018 18:24
Lead acid does normally not require balancing like lithium ion does, which makes them simpler to implement in stationary systems. Put as many in series as you want and set charge voltage to 2.25v per cell and it can stay that way indefinitly. No special control needed. Ideally you want to monitor it for faults etc but that's about it. An equalize charge once in a while may be a good idea too. Basically it's giving it a higher voltage.

Telephone exchanges run on 48v banks and the rectifiers are normally set to float at 54v. If power goes out the batteries take over load as they are part of same circuit. (kinda like a capacitor)

Of course with car batteries you want to avoid discharging too much but they should work.

#11 Reply
Posted by DougSpindler on 12 Apr, 2018 18:28
Something else to consider about batteries, efficiency. Dave had a video saying 20% of the energy that is used to charge a battery is lost as heat when charging. (Exothermic reaction.) And another 20% is lost as heat when discharging. (Another exothermic reaction.) So 40% of the energy used to charge/discharge batteries is lost as heat.

#12 Reply
Posted by Sixone on 26 Jun, 2018 05:21
Depending on the needs of each person is not equal.

#13 Reply
Posted by NorthGuy on 03 Jul, 2018 14:22
Car battery has very little capacity and won't last long. Perhaps they have Golf Cart batteries on Mars? Would be much more economical.

Other than that, you need a charge controller.

If you have higher voltage solar array, you need MPPT charge controller. It's essentially a buck converter. You'll need an inductor, two capacitors (in and out), a FET and a diode (or another FET instead of the diode).

With low voltage array, a cheap PWM controller may work, although it'll be less efficient than MPPT. Only one FET is needed.

#14 Reply
Posted by theoldwizard1 on 24 Sep, 2018 15:12
Some folks, who do not mind the labor, get automotive batteries from junk yards for a few dollars. They negotiate with the scrap yard so that if they find a dead cell/can not rejuvenate them they can just swap it for a different one.

These batteries will not have much life left in them and high charge/discharge rates will not help. At least once a week, all cell MUST be checked for adequate water/acid and specific gravity. Better have a couple of spares on hand because you will need them.

Also, you will need a way to dispose of the batteries when they are dead (some cities will recycle them).