A123 Lithium Pack Charger

[retepsnikrep]

I have decided to make my own onboard charger for my A123 20ah pack using 3 x 100w 48v switch mode psus. Using info from the endless sphere forum I have modified them to up the voltage slightly to a maximum of 60v output each. This gives 180v total which is around my maximum pack charged voltage.

I have also modified one of them further so I can control it's output voltage remotely.
This supply will help me balance the cells as they near full capacity.

The supplies mains inputs are connected in parallel and controlled by a mains relay driven from my BMS. If any cell goes over my max V then the power is cut.

The supply outputs are all in series.

The supply with the remotely controllable output voltage basically has two extra connections to which I attach an opto output from my BMS. The opto has to be driven by the BMS to allow the psu voltage to rise to the reqd voltage. i can drive this with a PWM output and therefore control the output voltage of the supply. As a cell reaches the balancing voltage I reduce the drive to the opto which lowers the psu output voltage and hence the current.

I have taken some pics of the mods so far and i'll post them when it's all in and working.

I may well make a higher power version using 350w supplies later but for the present I should be able to get about 2.5A from the 100w ones with a bit of luck. They are cheap as chips and fairly easy to modify.

[Cobb]

Psus is a power supply and the output is combines in series between the 3 units?

Originally Posted by retepsnikrep

I have decided to make my own onboard charger for my A123 20ah pack using 3 x 100w 48v switch mode psus.

[RobertSmalls]

I read the two EndlessSphere links you posted earlier with interest. I'm not comfortable wringing extra voltage out of a PSU, but controlling the current sounds very interesting and useful.

[retepsnikrep]

Originally Posted by Cobb

Psus is a power supply and the output is combines in series between the 3 units?

yes that's correct

[MN Driver]

I've been going back in forth in my head about going with one of the BMSbattery.com chargers for an e-bike using lithium and modding them like on endless sphere. After finding out that the bmsbattery.com ones aren't isolated and generate quite a bit of heat, I decided that if I want to charge with power supplies it will be with isolated ones that have more efficiency. It looks like doctorbass over there is pretty happy with his Meanwell 1500watt 48v output supply he has saying it is extremely small and efficient in the 9x% range but you couldn't connect those in series at that wattage without blowing a breaker, those come at a high price though. I'll be looking at 350 watters if I ever build a lithium car pack, which seems to be delayed quite a bit if I ever go that route since the biggest supplier that had the cheapest LiFePO4 delivered to the states decided to be a terribly bad apple and sour our affordable battery availability.

I'll have to go back and read up about raising their output voltages, I missed that part. I remember that they would usually cut one shunt out of three in the supplies though to drop the amperage to a thermally stable level though.

[retepsnikrep]

Been doing more work on this and am just modifying three new meanwell 350W 48v units to give 60v each.

In bought them from the below, came in about 10 days from china. Three chargers came to about $110 which is very cheap compared to a 'proper' EV battery charger. I paid over £500 for my last Zivan charger and it took two months to arrive

48v items - Get great deals on Tools, Prototype PCBs items on eBay.co.uk Shops!

They seem pretty easy to mod, with all the bits identified in the endless sphere forum thread in the right places on the pcb. I will post some decent picks of the assembled charger installed later and the mods in detail.

Just waiting for some 68V zener diodes to arrive to raise the output voltage. I should be able to get about 5-6A out of them at slightly over 350w. I'll use my watt meter to do some sums when it's all in and working.

The three supplies all have the following mods.

1) Fan mod to activate fan as soon as psu is turned on. Simple 470R resistor inserted instead of thermistor.

2) Raised output voltage mod. Simple replacement of 2k resistor with a 1.2k device to push up the operating voltage range window.

2A) Replacement of over V voltage detection zener with a 68v device to enable 2) above.


One of the psu's (Let's call it the Master) has two further mods.

1) Current limit mod to lower output current so total supply power does not exceed 350w or so. This is achevied by removing one of the three shunts on the pcb reducing output current by a third.

2) Output voltage control mod. This enables my BCM to control the output voltage by shorting out the V adjust pot to pull down the output voltage to the minimum for the psu.

All three psu's in series should run at the current as determined by the Master.

I should be looking at a cheap 1kw charger once finished, which weighs only a couple of KG and should fit in the ipu compartment on the A123 car. I might put them down the right hand outside of the IPU comp as there is a fair bit of free space there. I could mount them in a nice box and make it a portable charger to save lugging the extra weight about. Hmm? I would need a six way connector rated at about 8-10 amps.

The charger is controlled in three ways.

1) The total output voltage of the combined psu's is adjusted to the pack full voltage limit or a tad below. So they limit themselves when output voltage rises to the preset maximum. So if the BMS fails and the pack is reasonably balanced it should be impossible for them to overcharge the packk.

2) The BMS controls the output voltage of the Master Psu. If a cell goes over V it can cutback the voltage of the the master PSU reducing charge current.

3) The BMS controls the solid state mains relay which powers up all three Psu's. If a cell goes over V and it can not be corrected by 2) above then the mains can be cut.

[retepsnikrep]

Finished this now and it's in and working, I set psus to 5.5A output at 60V which is about right for the cells and allows the chargers to operate within limits. Gives a recharge time of 4hrs from flat for my 20ah pack and draws around 1kw from the mains when checked using my watt meter.

Couple of pics showing Stack of three psu's in IPU comp and solid state mains power relay controlled by BMS.

[MN Driver]

Nice job Peter! I'm a little confused though, if you are measuring around 1kw from mains and you have a 60 volt output each(180v) at 5.5 amps, that would be 990 watts total. I figure that the efficiency of these would push you at least a little beyond 1kw. ...or are you getting some pretty stellar efficiency with these?

It looks like they fit beautifully and if I decide to go with LiFePO4, this charging setup will very likely be what I will use.

[retepsnikrep]

I would say these are around 90% effcient in this configuration. Comments on the ES forum indicate effciency increases with higher output voltage which is nice. They don't get very hot in use. My watt meter and clamp on ammeter are not hyper accurate so 1kw is a ballpark figure. Perhaps it's nearer 1100w I'll measure it again but i haven't used the car for a few days. Too much other stuff to do.

I turned up the voltage slightly on these today to 61v each and added a 6amp 400V diode in the + lead to stop any reverse current flow when chargers are off. Previously the output active led in the chargers would remain illuminated when not charging indicating a small amount of current was flowing back through them from the battery.

[MN Driver]

Sounds better than I imagined, good to hear that everything is working smoothly with them and with better than stock efficiency and cool operation. If only I could get around 80 volts out of a single PSU, I could charge my Insight NiMh(scrap cells) battery-powered lawn mower very quickly. *scratches chin* ...but I'm only dreaming that would be quite a lot to ask of those and I'm sure the caps and other components would burn up at such an extreme voltage from their standard range. Cool to know that they can reach 61 volts though.