# Idiots guide to chargers



## Moltenmetal (Mar 20, 2014)

You have the advantage of 240 VAC at every plug in Australia. Bridge rectified, you can get 1.44x 240 V minus losses as a peak charge voltage without a voltage boost stage if I recall my math correctly. That means a cheap charger should be possible. 

Li-ion chemistries have very flat voltage vs state of charge curves on both charge and discharge. So you don't need an enormous voltage beyond your pack nominal voltage at the end of charge. As an example, my LiFePO4 cells have a nominal voltage of 3.2 V and trip the BMS high voltage cutoff at 3.68V. My nominal pack voltage is 105V, but the charger starts to go into constant current mode at about 112 V and (theoretically) shuts off the charger at 3.65V per cell- theoretically I add, because I have never managed to get every cell to 3.65V before one of them goes over 3.68V.

Really, all you need is a current regulator circuit- one which will keep the current constant at a limit your source can tolerate. It would be ideal if it were an adjustable current setpoint. That, plus a cell by cell BMS to shut off the charger when the first cell goes to its high voltage cutoff, and you're done- nothing fancier needed.

I have an ElCon charger, and it has a fixed current which varies with input voltage- about 12.5A from 120VAC and 25A from 240VAC- perfectly functional here in North America where your typical household circuit is 120 VAC 15 A and all the 240 V circuits for stoves, dryers and welders etc. are typically at least 30 A. It doesn't allow the current setpoint to be reduced to match the limit of the AC power source's circuit breaker as far as I know, but there may be other models or features available.

Based on my reading, I'm of the opinion that shunt charging in order to top balance the pack is a bad idea for the longevity of the cells- and that a BMS is pretty much a practical necessary. Others here manage to bottom balance and do without a BMS, but it would take a lot of experimentation and careful cell by cell voltage monitoring during charging to avoid exceeding the safe upper voltage limit on one cell or another during charging. Voltage rise near 100% state of charge is very rapid indeed. Cells are expensive enough that a basic BMS is what I would consider to be necessary insurance to protect that investment. A working BMS lets you charge without worry, which is worth some money.

If your pack is undersized such that 2-5% of your pack's depth of discharge greatly matters to you at the top end, then you may need a fancier charger with a more complex curve, and celltop BMS boards capable of shunt charging. The better solution to that problem is to add cells, if you can while staying within the voltage limits of your motor controller.


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## dougingraham (Jul 26, 2011)

Moltenmetal said:


> Others here manage to bottom balance and do without a BMS, but it would take a lot of experimentation and careful cell by cell voltage monitoring during charging to avoid exceeding the safe upper voltage limit on one cell or another during charging. Voltage rise near 100% state of charge is very rapid indeed.


It is easier to avoid a BMS than you think. After bottom balance you have a pack of cells where all cells are empty. You know that you want to stop charge when the first cell reaches somewhere between 3.45 and 3.55 volts. If you have an adjustable charger where you can select the CC termination point you start out by taking your cell count and multiply it by 3.45 volts. In my case I have a 52 cell pack so I initially set my charger to stop at 179.4. I started charging and because I had the current set to 5 amps I knew it would take around 20 hours to get done (100 AH cells). Wait 19 hours and then start looking at the cell voltages with a DVM. Find the cells that have the highest voltage. Keep watching those cells. If one of them reaches 3.55 volts measure the whole pack voltage. This might not happen before the charger turns off. Increase the voltage and start charging again. Repeat until you find the voltage you want to stop at. That voltage is now what you set in the charger as the termination voltage. No cell will go over 3.55 volts during a charge. I believe my charger ended up with the termination voltage set to 183.5 which is an average of 3.53 volts per cell. But I have no really weak cells in my pack so several cells shoot up at the same time at the end of charge. The voltage at the end of charge shoots up several volts within the space of a minute or two so charge termination detection is easy even with a bottom balanced pack.

I have not seen any cell drift in the three years I have been running this pack. I have essentially no CV period because my charge rate is right at the C/20 (5 amps) rate so when the charger gets to my selected voltage it just turns off. Keep in mind that you only have to do the bottom balance once. And you only need to find the weakest cells once on the initial charge of the pack and then you know which ones they are from that point forward.




Moltenmetal said:


> If your pack is undersized such that 2-5% of your pack's depth of discharge greatly matters to you at the top end, then you may need a fancier charger with a more complex curve, and celltop BMS boards capable of shunt charging. The better solution to that problem is to add cells, if you can while staying within the voltage limits of your motor controller.


Or buy the next larger capacity cell. If you are using 100AH you might choose to go to 120 AH if it is available. Or instead of 50 100 ah cells go with 100 60 ah cells and parallel them as buddies. You end up with the equivalend of a 50 cell pack of 120 AH cells giving you 20% more range.

Without shunt balancers on your BMS you still have to do at least one manual balance at either the top or the bottom. And if you have shunt balancers on your BMS it would still be a good idea to do one manual top balance before you let the BMS have a go.


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## SirNick (Jun 14, 2015)

Fellow newb, so take this with a grain of salt...

You see a lot of talk about programming and such because the community here appears to be a little more in the "Y" end of DIY. There are quite a few open-source projects where you can build your own charger, inverter, DC-DC, etc.

When you go shopping for commercial conversion kits and parts, you will usually need the dealer to configure the stuff for your application, then you leave it alone until such time that something changes enough to require a visit to your dealer again.

There's a point where you have to decide whether you like it "done and dusted", or if you want to have a soldering iron in one hand and the other typing lines of C code.


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## Elanimal28 (Nov 18, 2013)

SirNick said:


> Fellow newb, so take this with a grain of salt...
> 
> There's a point where you have to decide whether you like it "done and dusted", or if you want to have a soldering iron in one hand and the other typing lines of C code.


I love it.


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## itchyback (May 28, 2014)

I’ll keep reading what you’ve said, but it seems when talking chargers, things escalate pretty quickly, and this idiot is learning slow. 
Moltenmetal, I looked up bridge rectifiers, sounds good, but the most I can take from that is 240v @ 10amp is pretty good, I can get 15 amp pretty easily so it sounds like youre saying I shouldn’t be worried about a giant charger. 
I was hoping to avoid a BMS at this point, while it seems useful and good for peace of mind, I think the majority opinion is that they are unnecessary. My pack will be oversized for 90% of my travel but I wanted the flexibility of longer trips. 
Doug
Seems pretty simple and maybe I’m overthinking it. I might hit up the local EV club and see if I can look at an actual charger because my internet reading is probably making things more difficult. 
Sirnick
Lol, well said. I’m probably in the middle of that spectrum, happy to have a crack at most things, not coding though, I sucked at that at school. 

Any suggested pages for reading? EVTV episodes to watch or YouTube comparo’s to see?


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## Coulomb (Apr 22, 2009)

Itchy, a lot of the problem seems to be confusing input current (AC amps in the mains cord) and output current (DC current into the battery). Suppose your pack happened to be 120 VDC (for simplicity of the maths), and neglecting losses, you can charge at 20 A DC for 10 A AC input. That's because power in equals power out plus losses, so with a 10 A draw at 240 V, that's 2400 W, or 20 A at 120 VDC.

To be a bit more realistic: a 144 V nominal lithium pack (45 x 3.2 v nominal cells) is actually around 3.5 VPC near the end of charge (so that's 45 x 3.5 = 157 V). Suppose the charger is 90% efficient; with 240 V at 13 A, you'd have 3120 W, times 90% = 2808 W. So that's 2808 / 157 = about 17.5 A. Ay more than that and you will need a special socket for the charger to plug into.

So from a 15 A 240 V power point, 3 kW of charging will be about your limit; maybe a little more when the pack is still at lower voltage.


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## Moltenmetal (Mar 20, 2014)

Itchy: if you're just buying a charger, you just need to find one capable of the voltage you need to charge your pack and the current you need to recharge in the time you have available. Because you have 240V x 10A available easily, that is 2400 W which will likely mean your charger can put between 2200 and 2300 W into your cells without tripping the mains circuit breaker- if nothing else is on the circuit. A 2500W charger, unless it comes with a means to turn down the charge current, would trip your breaker, but wouldn't if you had a 15A circuit available to charge from. 

My point about the bridge rectifier was that your desired pack voltage was achievable without a voltage boost stage in the charger, which hopefully that would mean a cheaper charger for you than if your pack was say 400V.

As to the BMS- it depends on how much monitoring, and worrying, you want to do. Many succeed without it, but they tend to be people who don't mind hovering over their packs with a voltmeter at the end of charge checking individual cell voltages to make sure they're not prematurely ageing their lowest capacity cells. Just top balancing my pack was an anxiety filled event for me! If you want to plug it in and forget about it, the BMS is great- and at $15 per cell plus $50 for the headboard for the miniBMS, it's pretty cheap insurance on expensive cells.


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## itchyback (May 28, 2014)

Ok so, charging a 144pack with 240v 10amp input, the charger with magically change the output power to 157v and 15amps ish. ON a 288v pack the charger would need to supply 315v and therefore reduce amps to 7ish.

A bigger charger, say 30amps, in a circuit that could supply that amps (at 240v) could charge a 144v pack at output 46amps ish and probably fully charge it in about two hours. But a 288v pack, the charger would have to fiddle with the amps (reduce to 25) to increase the volts to 315, to fully charge the pack. 
The problem then is having a circuit that could supply that, either stealing from the stove circuit or its special own circuit. Plus the cost of the charger and possibly involving a bridge rectifier (i think that would be in the charger, not something additional to buy)

Gee I hope that’s right, it seems right to me. 

Next question, 
How does the charger know when to stop?
When you buy an appropriately sized charger for you pack and household volts, do you plug it into your computer when it arrives and tell it how many volts to stop at (like entering numbers into a excel spreadsheet) or is it more complicated than that? OR do they come with knobs and dials and displays so you can set it to stop charging when it gets to 157v. (this assumes that there is not much change in voltage drift with repeated charging and discharging, which according to Doug and my broader reading is true). Or does the manufacturer arrange that sort of thing. 

If you had a BMS, it seems you could take the charger out of the box, plug in your power and the BMS would tell it when to stop when the first cell in a bottom balanced pack reaches about 3.5v, Borderline idiot proof, is that right?


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## Moltenmetal (Mar 20, 2014)

Forget about the bridge rectifier, it's part of the charger. That was just a quick check calc that I did to see if your charger would need a boost stage, which it probably wouldn't.

The charger I have comes with eight preprogrammed "curves" for different numbers of cells, and of course was set up for my 32 cell pack by the vendor. But those curves are limited to a certain maximum voltage based on the supply voltage.

My charger has a complex " curve", meaning that it operates at constant current as the pack voltage climbs, then switches to constant voltage above a certain threshold voltage, then changes to a low constant current to allow a BMS with shunt resistors to top balance the pack. At a particular voltage, in my case an average of 3.65 V per cell, it turns off.

The charger generally works by converting AC to DC using diodes (arranged in a bridge configuration, hence the name), and then using transistors to turn that DC on and off really quickly to regulate either the current or the voltage to a particular value. Capacitors are used to flatten out the peaks of the resulting chopped up DC so your cells don't see very high voltage spikes. There's more to a switch mode power supply than that, but that's a thumbnail sketch from an amateur's perspective. If the voltage of the DC isn't high enough, an inductor is fed the chopped DC and used to generate spikes of higher voltage. That's the boost stage I was talking about.

Since I have a BMS, I didn't need a sophisticated charger. I just have the BMS stop the charge when any cell exceeds the high voltage cutoff, which is 3.68V or so. It rarely gets up to the voltage that it switches to constant voltage mode before that happens, but I know from testing that at that point I only have a few Ah to go before I reach 100% state of charge on all but a couple of my 32 cells. I'd rather give up a few Ah out of 180 rather than hold my cells at a potentially damaging high voltage any longer than necessary.


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## PStechPaul (May 1, 2012)

One important point that has not been covered is power factor, and a simple, cheap charger without PFC may draw the maximum 10A at 240V and yet only be able to deliver about 1200-1500 watts. This phenomenon is separate from efficiency, which for most decent chargers is greater than 90%. You can buy a charger with PFC for a premium price, but if it allows you to get the full 2400 watts then it may be worth it.

Poor power factor is caused by the bridge rectifier and the main capacitor bank drawing high current during voltage peaks, and these peak currents can exceed the breaker rating and cause it to trip. A PFC circuit uses a boost converter which draws input power during the portion of the AC input where it is at a lower voltage portion of the sine wave, and limiting the current near the peaks, but drawing the same amount of power. It's too complicated to explain to a non-EE type, but it's not rocket science, either. It is just important to know what PF is and why it can be important to have PFC.

If and when I get far enough into the EMW charger project I may make a separate PFC module which could be connected to a non-PFC charger. A 10 kW PFC should cost somewhere around $500-$1000 (or less) for a mostly DIY kit.


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## EVsonic (Aug 14, 2014)

Hi

Interesting thread

I have just sent my kingpan charger back after no more than 10 charges keep flicking on and off like it couldn't see a load.

Anyway it got me thanking I am on stand alone solar and thought it may be worth do the following.

Buy 10 260 watt solar pannel that would give me around 159 volts at 17amps
EV works in Perth have done some direct charging from panels.

Buy 20 260 watt panels this would give 34amps. For about $8200

I want to know the following.

Solar controller at 159 volts cant find or can the BMS kick off I have a relay on the Anderson plug to do this on my 240volt charger

I have 46 x 160 ah batteries

How long to charge at 17 amps and at 34 amps?

And I thought I could buy a mains feed inverter and when I am not charging car I could plug in my 48 volt 40 amp charger to help charge house batteries.

Any help would be great.

Cheers Kiwi


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## itchyback (May 28, 2014)

I read somewhere and i cant remember now. I thought it said amps in equals miles out or something thereabouts. That doesnt really make sense now i think about it.

PStechPaul, sorry, you lost me after "one important point....." 

I think i have the gist enough to have a crack. Thank you everyone.


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## SirNick (Jun 14, 2015)

The EVTV guy gives a rough rule of thumb that 100Wh in per 1000lbs of car will get you 1mi out.

Seems like as reasonable a starting point as any. You won't know for sure until you build, test, and measure.


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