624V of SLA chargers

[e-ghia]

Hey guys, once again I seek advice from the AC high-voltage gurus.

I mentioned in another post that we are engineering students, but what I should have said was mechanical engineering students.

We're ordering 52 SLA Greensavers from China, and we'll probably get their 48v chargers also (as they're giving me a decent price). They are 5A IC controlled chargers. My question is this: how can I operate 13 chargers without tripping my circuit breaker (on the wall) all the time?

I am in the US and we'll use 220VAC when available, but will often have to use 110VAC for 'opportunity charging.' Most of the 110V breakers are 20A. Is their some way I can throttle back the AC current going into the chargers? What are the pitfalls? Should we consider some sort of timers or sequencers instead?

What do the others with 600V systems do (or plan to do)? As always, any advice is greatly appreciated.

[woody]

5A 48V = 240W / 80% efficient = 300W per charger at the wall.
110V 20A = 2200W.

The 20A fuse won't really blow at 20.05 Amps, so you can probably do 8-10 chargers at once.

If you have 48 Cells in the car you could arrange it so that each 6 of the chargers charge a 2x4 block of 8 cells at 48V, taking twice as long.

If you have 220V 20A, then you can charge using all chargers?

[a4x4kiwi]

You will need to specify the charge current based on what is available at the wall.

If you have 220v 10A that is 2200W (same W for 110)

So, if the chargers are 80% efficient, you have 1760W to play with.

If you have 624v and 1760W the max charge current for batteries in series will be 2.8A. before overloading the wall socket. You could probably get away with 3 A chargers.

[Johny]

Is their some way I can throttle back the AC current going into the chargers? What are the pitfalls? Should we consider some sort of timers or sequencers instead?

You can't actually wind back the AC with anything other than counter-productive results - the chargers will draw more current at lower voltages.

Woody's suggestion about using less chargers could be a good way to go but it means being able to parallel up 48V battery banks (only during charging) so that you can charge 2 banks with one charger.

The other way, as you suggested, is to sequence the chargers, say 30 minutes on, then the "other half" get 30 minutes - then around again. I have a feeling that this may not be the best thing for the batteries.

If you are stuck with these chargers then I would try to find someone who might be able to "hack" the chargers to reduce the output current. If they can be opened up this should be a fairly reasonable ask for someone who understands Switch mode power supplies.

[acmotor]

You do need to confirm the input current to the chargers i.e. are they power factor corrected so does the 300W draw only 1.36A off 220V or more likely a pf of .9 (or less) so 1.5A (or more).

Thus the 13 chargers could draw 19.5A or more off 220V or 39A off 110V.

The second factor is the inrush current at turn on.
Most SMPS of 300W can pull 30A at turn on so the kick of 13 of these may blow the fuse already. Some additional limiting of inrush may be required.

The though of sequencing the chargers may be helpful. However steady lower charge current would be preferred with SLA ?

Agreed, cutback without charger internal mod is not practical.

Mal, how do you get on with your chargers ?

I run 1 to 12 chargers (48V modules can be paralleled as required).
6 seems best as a full charge in less than 12 hours on SLA is not a full charge and stresses intercell balance !

I'd ask for the chargers to be throttled back at factory (or at least get the details of how to do it yourself). After all, you could fit a 3 phase plug as an option !

[Nevilleh]

Sounds like you are running one big series chain giving 624V rather than what I have seen others do, which is to split the pack in the middle to give +- 312V. I surmise this as you have 13 chargers, presumably 48V/5A. The charger outputs are all in series, in banks of 4 batteries per charger - I wonder if their insulation to ground is up to it, but if everything is isolated should be no problem. But you could just run 7 of them until the batteries they are connected to are charged and then run the other 6 likewise. 7 should only draw 20 amps from your 110V supply. Or, if you have 220V available, run the whole 13 at once for only about an 18A load.

[a4x4kiwi]

My chargers are only 2.5A output and draw about 7A from 240V mains. If I were buying again I would go to 3A.

Mal.

[e-ghia]

Fantastic input as always! Thanks guys.

The design consists of 13 x 48V modules separated by contactors, each with its own charger. As a4x4kiwi suggested, we'll probably go with the 3A chargers. They are half the price and rated at 180W. 13*180 = 2340W which still may trip the breaker. The 20A breaker is rated for 2400W, but from what I read we should only load it to 80%.

Even if the breaker can handle it, acmotor brings up a great point about inrush current. My hope is that the inrush is brief enough that the metallic strips in the breaker won't have sufficient time to heat up and break the circuit. Any thoughts? What can be done about inrush current?

We can always charge by 220VAC from the wall instead, and we can reconsider fewer chargers with parallel modules or 2A chargers after testing it out.

Sounds like you are running one big series chain giving 624V rather than what I have seen others do, which is to split the pack in the middle to give +- 312V.

Yeah, I see that in acmotor's and a4x4kiwi's designs, but I don't understand why or even how it works. I'm a VFD novice, can someone explain this +- string? What's the advantage? How is this configured?

Thanks as always.

[Nevilleh]

The design consists of 13 x 48V modules separated by contactors, each with its own charger.

Yeah, I see that in acmotor's and a4x4kiwi's designs, but I don't understand why or even how it works. I'm a VFD novice, can someone explain this +- string? What's the advantage? How is this configured?

Can't see why you need to separate each 48V bank with its own contactor, shouldn't be necessary - at least not for charging. Seems to me to be a waste of money!

The idea of splitting the battery pack is so that you only have +- 312V each side of "ground" which is less stressful than 624V.
The ac motor controller used to be fed with ac which is also +-, so its keeping things the same.

[acmotor]

Human Safety.
The 48V (or at least something less than maybe 72V) and totally floating modules when contactors open, makes them far more safe to work on. Tried spannering a battery pack lately ?
This is of particular interest in conversion EVs where the batteries are more than often distributed around the vehicle.

The module arrangement also allows various charging options and means that the EV is not sitting around (on charge) with the 600V ready to zap !
With the key out of my EV there is no more than 48V anywhere. I like it !

[Squiggles]

Human Safety.
The 48V (or at least something less than maybe 72V) and totally floating modules when contactors open, makes them far more safe to work on. Tried spannering a battery pack lately ?
This is of particular interest in conversion EVs where the batteries are more than often distributed around the vehicle.

The module arrangement also allows various charging options and means that the EV is not sitting around (on charge) with the 600V ready to zap !
With the key out of my EV there is no more than 48V anywhere. I like it !

I fully support you drive for electrical safety, 48V break up is a bit excessive though if the general installation is designed for safety.

After all the toaster in you kitchen is a whole lot more dangerous.

Still it must be easier to sleep knowing that no damage can be done by your set up.

[Nevilleh]

Yes, fair enough - the only thing I'm paranoid about is that I might not be paranoid enough

I think breaking it up into 48V modules is a bit excessive, but if it makes you happy, why not?
I have a couple of spanners I use on my battery pack (only 150V but still lethal) and I have covered them with heat shrink tubing so the only exposed bit is the ring end. I also have some rubber coated gardening gloves and I tend to wear them when I am fiddling with the battery pack, just in case.
You can't be too careful when dealing with high voltage, dc particularly, but you can still wire 13 48V chargers in series!

BTW my pcb is half assembled, done all the .5mm lead spacing bits.

[acmotor]

That's fine.
48V may be low but also comes from my concern about cell balance with lead acid.

I trust you can see the point that if you have a 144V battery pack then breaking it at least in half to 2 x 72V isolated modules (and even charging them in parallel) makes a whole lot of sense safety wise. It really doesn't cost much.

If you have a 600V pack, don't even think of working without breakup ! Insulated spanners or not.

[antiscab]

other advantage of 48v modules means you can use those cheap 16 cell BMS modules that go for US$80ea to do balancing and over-charge prevention.

i know 50ma shunt is enough for TS 40 AH cells :p

Matt

[woody]

other advantage of 48v modules means you can use those cheap 16 cell BMS modules that go for US$80ea to do balancing and over-charge prevention.

Where can I get one of those? (er, 15 of those)

[antiscab]

16 cell 30A BMS

need to add fuse to the two heavy wires, the light ones are light enough they're they're own fuse :p

over-discharge warning could be trickier to setup.

Matt

[acmotor]

Mmmmmmm, I'd be looking at Nevilleh's 8 channel BMS work in progress.
That 16 cell BMS is a toy.

[a4x4kiwi]

FYI here are some nice Lead Acid 2.5A 75V chargers from e-crazyman where I got my previous chargers and power supplies.Here

[acmotor]

Yes, my 2A 55V 'chargers' came from e-crazyman.

These 72V are presumably for 6 x 12V SLA so they go to 90V peak (~15vpc) if they are intelligent ?

[EV2Go]

Yes, fair enough - the only thing I'm paranoid about is that I might not be paranoid enough

I think breaking it up into 48V modules is a bit excessive, but if it makes you happy, why not?
I have a couple of spanners I use on my battery pack (only 150V but still lethal) and I have covered them with heat shrink tubing so the only exposed bit is the ring end. I also have some rubber coated gardening gloves and I tend to wear them when I am fiddling with the battery pack, just in case.
You can't be too careful when dealing with high voltage, dc particularly, but you can still wire 13 48V chargers in series!

BTW my pcb is half assembled, done all the .5mm lead spacing bits.

I’m with you Neville... I would rather be alive and paranoid then dead and blasé. Guess it is my electoral ignorance that keeps me paranoid, who said a little knowledge is a dangerous thing

[antiscab]

That 16 cell BMS is a toy.

well it is commonly used in toys

it is rather basic in design, but it does the job, namely balancing and overcharge prevention (the 30A model really will handle a 30A charge rate).

I know the failure mode of these BMS's do have a safe outcome (destructive testing reveals all )

and when its time to upgrade, you can upgrade to Nevilles new wiz bang 16 cell BMS modules, safe in the knowledge your first BMS didnt cost all that much.

Matt

[Johny]

Yes, my 2A 55V 'chargers' came from e-crazyman.

These 72V are presumably for 6 x 12V SLA so they go to 90V peak (~15vpc) if they are intelligent ?

I emailed him a few weeks ago to ask what the maximum voltage for the 60v 2.5A charger was and he replied 69V. If you extrapolate from that, then the 75V is around 83V max (or 13.8V per batt).

[acmotor]

Good info.
So they are just float charger CV 13.8vpc CC 2.5A

BTW
My feeling so far with my SLAs is that the charge current should be dropped back at around 13.4vpc to around 100mA on 20Ah cell and held for 8 hours at CV 13.8V CC 100mA to get the last of the charge in. ( battery will rise to 13.8V (temperature corrected of course) and only mA of final current.
Any faster (without individual 2V cell balance/shunt) and you will force some cells >13.8V detrimentally. (RE doesn't work at Amps of charge so battery will gas (vent) and lose water) Place your ear to one if you want to check.

I know the SLA 'overcharge' technique of going to ~14.5V to eq the cells but this also ignores the intercell unbalance that exists when batteries start ot age.
Pulse chargers do better here, but tests I've seen still show loss of water. (probably the main cause of failure in SLAs)

IMHO, with Lithiums, the final charge current should be less than the shunt eq current and just slightly less than the shunt voltage for the string. The voltage monitoring BMSs will be helpfull here.

[Mesuge]

Good info.
So they are just float charger CV 13.8vpc CC 2.5A

So, are you sure that e-crazyman sells 48V ordinary float chargers, which at first glance appear as at least multi-stage(3) "smart" chargers? Well that should have rang some bells, as they are usually 5-10x more expensive. Perhaps you can get the same effect at lower ripple plus other goodies (higher efficiency PFC, PLC input, ..) by running SMPS instead (range: 40-3840 Watts)
http://users.tpg.com.au/users/p8king/s100.htm

But I'm not sure about the long term effects (bad) of float charging (even performed around <0.1C)
for cyclic application (<80% DoD) as done with EVs..

[acmotor]

My understanding of multistage cyclic charge of SLA by going to 14.5V is that it allows charge in 4 to 6 hours whereas simple cv float charge with current limit of 0.1c or less requires 16 hours charge.
The faster charge is achieved at the expense of cell water if the RE system can't keep up as the cells lose balance.

An SLA left on float charge can expect a life of 10 years. So float is in no way detrimental (even less so if it is temperature corrected or at least within temperature range) i.e. actual float at 13.5 to 13.8V with tc of -20mV/°C (depending on manufacturer) and not float "overcharge" where cc is applied rather than cv.

I'd expect 3 years in an EV application as the hundreds of cycles come up.

I don't see that rushing the recharge has anything to do with the discharge cycle.

The e-crazyman SMPS I use are nominal 48V adjustable 45 to 57V with 2A nominal(actually 3.18A) current limit. (they are not battery chargers)