Now that we know the goal is EV charging one question comes to mind: Have you calculated the charging watt-hours goal (i.e. to the EV) and how does that compare to your expected battery bank watt-hours? To simplify assume the battery is at a constant 12V, and allow for some small percentage loss (5%?) in the electronics and charger. (We know the battery isn't fixed at 12V, but the intent is a quick determination if your approach is even practical.)

 

So I surmised; so why I was (half-heartedly) considering a trio of 6v cells running at a "less depleted" ~5v or so: Just under 15v doesn't ostensibly upset the inverter's over-voltage cut off <fingers-crossed>.

That's still not the right answer. You seem committed to doing this wrongly, one way or another.

With Pb cell sag, most deep-cycles drop below 12v quickly

They don't if you're maintaining a reasonable discharge rate per battery.

the inverter that it's powering is intolerant of over 15v nor under 11

This is a reasonable restriction and is designed to prevent damage to your batteries. If your 12V battery is below 11V then it's dead. Yeah, there is still more juice that can be squeezed out of it, but at the cost of permanent damage to the battery. You should not discharge a 12V battery below 11V or a 6V battery below 5.5V.

Let's say you find the exact 150A stepper-upper thing that you're looking for, to put between your battery and inverter. Once your battery voltage drops to 11V, the stepper-upper keeps delivering 12-13V. And once your battery drops to 10V, inverter still going. 9V, still going. 8V, still going.. How low do you intend to go? 0V? You want to kill the battery permanently in just a handful of charge/discharge cycles and then go out and buy a replacement? If so, why not just go out and buy the next 5 replacements ahead of time and wire them in parallel now, without the hypothetical stepper-upper, so that you can operate the inverter for a sufficient amount of time in the manner in which batteries and inverters are actually meant to operate?

Please re-read:

As I mentioned in post #4, the 12V battery should never be discharged to 11V and a 6V defiantly to 3V. That is the way to kill the battery very quickly.
You really are trying to do this the wrong way.
Either you need a number of new 12V batteries in parallel to increase the capacity, (not just one or two batteries but many for that load for more than a few miniuts. For batteries, more is better I think! ) or a similar number in series to boost the voltage and go to a higher voltage inverter. To try to draw 150Amps from a car battery for any length is a great way to kill it.
It is like you are trying to run a car on a lawnmower motor.

We are not trying to be nay-sayers here. Just trying to help you arrive at a solution that will actually work.

 

Powering an EV charger from ANY battery powered inverter is terribly inefficient. Is there no mains connection in the area?
How about an entire DC to DC scheme, rather than powering an existing charger? That could at least provide some trickle charging.

Here I see the basic fallacy of the EV concept, which has very deliberately ignored the issue of recharging. That the energy ultimately must come from someplace.

 

Yes, it is way out there to try to charge an EV from a 12V battery. It makes very little sense. Also, unless you had a stack of 12V batteries, there is really no point as the amount of energy available from one battery is pretty limited.
(I wish I had an EV here, my 10KW solar would make running one very cheap indeed!)

 

How would those 12 volt batteries be recharged? And how would they be transported from the charging location to the EV charging location? 12 volt batteries of large capacity are usually quite heavy and also rather expensive. The fact is that they need to be transported from the charging location and then transported back from the use location to be recharged.
So can the TS explain the rest of the situation? The collective knowledge here may be able to arrive at a solution, the resources ofso many are, in total, very large.
The solution mentioned by Dendad would certainly be best, but solar charging after dark is seldom effective.

 

How about using a series of dropping resistors (heavy gauge wire for 150 amps) and a batch of contactors to act as shunt relays (starter solenoids?) actuated by comparator-ladder network to switch out the segments one by one (maybe 1/2 volt each) to maintain a reasonably level voltage input to the inverter? Crude, but potentially very effective.

I might just look into nicrome wire resistance properties. As manual intervention is not out of the question, I could pretty readily switch out/in lengths of heating coil as needed. That’s the frugal creativity I was hoping for!

 

How long do you need to be able to draw 150A from your batteries? Realize that even with deep discharge lead acid cells, you can't use more than 50% of the batteries capacity or you will kill the battery.

In my opinion, you are dismissing LiPO4 batteries too quickly since they eliminate your problem entirely. Lithium batteries have a very flat discharge curve and the output voltage varies from 12V (10% of capacity) to 12.8V (Fully charged). Lithium cells are also capable of output an astounding amount of current. For example, Headway 38120 cells are typically rated at 8AH, but are capable of sourcing 200A for short periods of time (~2 minutes). The price of prismatic cells has also come down a lot in the last year. For example 3.2V, 280AH LiFePO4 cells are now selling for $65 each on Alibaba. I built a 25.2V, 280AH battery for my RV out them, and they were $100 each 2 years ago.

Certainly that technology is the front runner, but to get to at least 100 amp hour worth of 12 V or 24 V, for a half hour, it is perhaps double what I’m hoping to spend with bits laying around. Thx tho’ (And that is certainly the eventual solution when prices continue to drop)

 

How would those 12 volt batteries be recharged? And how would they be transported from the charging location to the EV charging location? 12 volt batteries of large capacity are usually quite heavy and also rather expensive. The fact is that they need to be transported from the charging location and then transported back from the use location to be recharged.
So can the TS explain the rest of the situation? The collective knowledge here may be able to arrive at a solution, the resources ofso many are, in total, very large.
The solution mentioned by Dendad would certainly be best, but solar charging after dark is seldom effective.

Essentially, as you see, I am building a UPS. The expectation is that it would recharge it’s bank from home mains periodically. Ultimately (to realize more intelligence and adjustability) I will probably try to locate a scrapped APC unit and repurpose it.

 

As I mentioned in post #4, the 12V battery should never be discharged to 11V and a 6V defiantly to 3V. That is the way to kill the battery very quickly.
You really are trying to do this the wrong way.
Either you need a number of new 12V batteries in parallel to increase the capacity, (not just one or two batteries but many for that load for more than a few miniuts. For batteries, more is better I think! ) or a similar number in series to boost the voltage and go to a higher voltage inverter. To try to draw 150Amps from a car battery for any length is a great way to kill it.
It is like you are trying to run a car on a lawnmower motor.

Pursuant to an earlier reply, since I am essentially creating a UPS… I know expect to acquiesce and repurpose a high wattage APC unit surplus. Those readily run on two pair of 12 V gel cells; that aligns with an earlier reply that supports 24v/48v dc-in. Thx!

 

Essentially, as you see, I am building a UPS. The expectation is that it would recharge it’s bank from home mains periodically. Ultimately (to realize more intelligence and adjustability) I will probably try to locate a scrapped APC unit and repurpose it.

Building a UPS (Uninteruptable Power Supply) for an EV charging system does not seem like a very useful thing, unless your power is frequently interrupted. For the hopefully rare occasions when the mains service is unavailable it will be much more cost effective to use a fuel powered generator system to directly power the vehicle battery charger. UNLESS YOU ARE IN CALIFORNIA! If you are indeed located in California I have no acceptable advice to offer.

 

@moapys I think you would benefit from reading a bit about the Peukert effect. It describes what you are experiencing. When you apply a high current load to a battery (especially to lead acid batteries) the amp-hour capacity goes way down. If a battery has a 100AH rating it should be able to deliver 1A for 100 hours but that doesn't mean it can deliver 100A for 1 hour. With a 100A load it might last only 30 minutes, and then it will be <10-11V and therefore dead. You feel cheated because you only got 50AH out of a 100AH battery and from reading this thread it seems like you feel that the missing 50AH is still in that battery somewhere, just inaccessible to your inverter because of its unreasonable restriction to 11V. It seems you feel that this some kind of phantom effect you refer to as "voltage sag." Let me assure you that this decrease in capacity isn't some imaginary thing. It is very real and once the battery gets down to 11V it is dead. Coming up with some clever means of further discharging it to recover the missing 50AH is NOT going to give you any appreciable amount of additional runtime (because that 50AH is in fact no longer in the battery) but it IS going to permanently damage the battery.

The proper solution is to put several batteries in parallel such that each battery must contribute only a fraction of the high current load, and therefore experience less Peukert effect, and deliver its proper number of amp-hours. Doing this will give you much higher total AH capacity, plus will allow each battery to actually deliver its rated AH capacity.

No "creative solutions" exist to side-step the physics going on in this scenario, and anything being discussed here is just going to make the situation worse. If you add a half-charged battery in series and then drop the voltage down with resistors, diodes, or too-small wires, then a bunch of heat (a hazardous amount of heat) will be generated in those resistors/diodes/wires. Heat is waste energy. If there's anything you don't want to do when you're already trying to overcome a deficit of power, it's waste more power. If you put a boost converter between the battery and inverter, it too will waste power; you can only expect about 90% efficiency in the boost converter at best, and so your batteries will go flat 10% faster. And if you discharge them below 11V routinely they will have a very short life.

Get more batteries, put them in parallel, and cross thing off the list. If you want to share what batteries you're using or are interested in using, I would be happy to help you work out how many would be needed.

 

@moapys I think you would benefit from reading a bit about the Peukert effect. It describes what you are experiencing. When you apply a high current load to a battery (especially to lead acid batteries) the amp-hour capacity goes way down. If a battery has a 100AH rating it should be able to deliver 1A for 100 hours but that doesn't mean it can deliver 100A for 1 hour. With a 100A load it might last only 30 minutes, and then it will be <10-11V and therefore dead. You feel cheated because you only got 50AH out of a 100AH battery and from reading this thread it seems like you feel that the missing 50AH is still in that battery somewhere, just inaccessible to your inverter because of its unreasonable restriction to 11V. It seems you feel that this some kind of phantom effect you refer to as "voltage sag." Let me assure you that this decrease in capacity isn't some imaginary thing. It is very real and once the battery gets down to 11V it is dead. Coming up with some clever means of further discharging it to recover the missing 50AH is NOT going to give you any appreciable amount of additional runtime (because that 50AH is in fact no longer in the battery) but it IS going to permanently damage the battery.

The proper solution is to put several batteries in parallel such that each battery must contribute only a fraction of the high current load, and therefore experience less Peukert effect, and deliver its proper number of amp-hours. Doing this will give you much higher total AH capacity, plus will allow each battery to actually deliver its rated AH capacity.

No "creative solutions" exist to side-step the physics going on in this scenario, and anything being discussed here is just going to make the situation worse. If you add a half-charged battery in series and then drop the voltage down with resistors, diodes, or too-small wires, then a bunch of heat (a hazardous amount of heat) will be generated in those resistors/diodes/wires. Heat is waste energy. If there's anything you don't want to do when you're already trying to overcome a deficit of power, it's waste more power. If you put a boost converter between the battery and inverter, it too will waste power; you can only expect about 90% efficiency in the boost converter at best, and so your batteries will go flat 10% faster. And if you discharge them below 11V routinely they will have a very short life.

Get more batteries, put them in parallel, and cross thing off the list. If you want to share what batteries you're using or are interested in using, I would be happy to help you work out how many would be needed.

Actually, THERE IS a "creative solution" which would avoid the issue completely. That would be to have a series string of the 100AH batteries that could supply the charging voltage directly to the EV charge point. Without any conversion circuitry the efficiency can approach 100%, and in addition, the reliability will be quite high.
Besides that, the same battery stack can supply an on-demand inverter to provide house hold power during a power outage. Of course, a decision would be needed as to how much EV charge versus how much house-hold power to provide.
And a question: is there actually a need to a UPS for EV charging??. I would guess that most folks would rather not discharge their EVs that far, but rather always top-off their charge at home, because that is the least costly option.

 

Actually, THERE IS a "creative solution" which would avoid the issue completely. That would be to have a series string of the 100AH batteries that could supply the charging voltage directly to the EV charge point. Without any conversion circuitry the efficiency can approach 100%
[...]
is there actually a need to a UPS for EV charging??

Why are you looking at the forest when there are so many pretty trees around? Trees must be usually be chopped down one at a time until the forest is cleared. You're moving too fast.

Besides that, the same battery stack can supply an on-demand inverter to provide house hold power during a power outage. Of course, a decision would be needed as to how much EV charge versus how much house-hold power to provide.

That's the only way any of this makes sense for me at all. Using a 12V deep cycle lead acid battery to charge an EV? Nah, that should have been my red flag but I jumped in anyway. If you had a battery bank that was larger than the capacity of the EV itself, ok. But otherwise, using the EV itself as a UPS to power the house has more merit and some EVs actually have provisions for this.

 

Why are you looking at the forest when there are so many pretty trees around? Trees must be usually be chopped down one at a time until the forest is cleared. You're moving too fast.


That's the only way any of this makes sense for me at all. Using a 12V deep cycle lead acid battery to charge an EV? Nah, that should have been my red flag but I jumped in anyway. If you had a battery bank that was larger than the capacity of the EV itself, ok. But otherwise, using the EV itself as a UPS to power the house has more merit and some EVs actually have provisions for this.

I already asked the question about need, and I already commented that mains charging was a better choice, and I already suggested a fuel powered generator supply. So a whole voltage direct charging arrangement was another option. Horribly expensive and terribly inconvenient to move, though. And I never said it was a good idea.
And as for EV house power, my neighbor has a pickup that can supply power. Unfortunately it will not supply adequate power to start a (Natural gas heated) cloths dryer. But I think it was adequate for the refrigerator.

 

I already asked the question about need, and I already commented that mains charging was a better choice, and I already suggested a fuel powered generator supply. So a whole voltage direct charging arrangement was another option. Horribly expensive and terribly inconvenient to move, though. And I never said it was a good idea.

I see that my written sarcasm skills still need more work. I wasn't disagreeing with anything you said.