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DaveAK
02 January 2011, 1900
So it occurs to me that I haven't really given this part much thought. As you may or may not know I'm using a home made offboard CC/CV charger. Just like any other charger you connect positive to positive and negative to negative. But if the charger isn't on and I plug it in to the battery I'm going to get current flowing from the bike to the charger, right? I'm not sure if this is good for my charger or not. So here's a couple of questions.

Can I put a diode in the circuit so I only get juice flowing from charger to battery and never the other way around? What would be a suitable diode for a 100V 15A charger, and where would you place it? Something doesn't seem right about this to me, but my brain's already fried today so I can't think straight.

What if anything will blow up if I have the charger on at a couple of volts higher than a full pack, and then plug the battery in to it? Somehow I've got to be able to plug this in safely and be able to switch the charging on and off. The charger itself will switch off when it reaches HVC, but this still leaves it connected to a now fully charged pack. I could throw in another relay and break the circuit when charging is complete if the diode idea doesn't work.

Help. :o

magicsmoke
02 January 2011, 2102
The diode idea's fine. One in series with either the pos or neg lead.
Pos lead, cathode to bike
neg lead, anode to bike
At 10A though, the diode is gonna dissipate anywhere from 5 to 10watts so you'll probably have to heatsink it.
Also, pick a much overspecced one, a 200V / 50A is practically the same size / cost i.e. bugger all.

Can't answer the jumping the charger onto the pack question without knowing the charger o/p stage and sensing.

If for whatever reason you wanted to ensure that the pack was connected before the charger was turned on, you could use a 4-pole connector with 2-poles as a switch. i.e. link two of the poles together in one of the connectors. When plugged together they complete a circuit and turn your charger on.
Be sure to use a connector with progressive contacting though otherwise it defeats the object.

DaveAK
02 January 2011, 2138
Looking for diodes is giving me grief. Is a "transient voltage supressor" suitable? It's the most common kind, but I wouldn't have classified this as transient. Any chance that you could pick out a suitable item for me that I could then cross reference. :D

BaldBruce
02 January 2011, 2208
Pick a Shottky style diode so your power losses wil be lower. Definetly need to heatsink, but be carefull because alot of the exposed metal diode parts are actually electrically connected to the cathode. so you need to heatsink , but use a platic washer and nut to keep the electrical isolation. Or provide a heatsink that is not attached to other metal parts of your bike. Here is a Shottky diode example from Newark http://www.newark.com/stmicroelectronics/stps20120d/schottky-rectifier-20a-120v-to/dp/37M6762.

magicsmoke
02 January 2011, 2217
No a transient voltage supressor's no good for this.

Something like 1N1186A looks good. 200V / 40A $5.30
Convenient mounting to heatsink (be aware that the diode body in this case is connected to its cathode so don't short it to anything unless intentionally)
http://www.alliedelec.com/search/productdetail.aspx?SKU=5030587

Anything along these lines will do, nothing fancy speed wise etc.

If you want to look for similars try 'power rectifier diodes'

As for your general concerns re jumping the charger onto the pack, I think BaldBruce is probably your man to answer that sort of stuff. My (cough cough) speciality is microphone amplifiers and I'd hate to give you a bum steer.

Edit : Amazing! Just think of the powermeister and he posts before I even finish typing :)

DaveAK
02 January 2011, 2218
Thanks Bruce. I thought I needed a Schottky diode, but couldn't find them at first, and couldn't spell it either. :D I'm now going through the multitude of search options trying to find something suitable.

What are the chances of finding one at Radio Shack?

BaldBruce
02 January 2011, 2219
Personally I use this one because it is isolated and high enough voltage and current for my application.. 100V and 10A http://www.newark.com/vishay-general-semiconductor/mbrf10h100-e3/schottky-rectifier-10a-100v-ito/dp/65K1730 0.7 Vf worst case and 5.8 C/W is fine for my 5A charger, but would need a decent heatsink for a 10 A system

Let me know your exact maximum charge voltage and maximum current and I can fine tune a selection for you if either of these suggestions is not to your liking.

BaldBruce
02 January 2011, 2220
I wouldn't know about Radio Trash. ;-)

DaveAK
02 January 2011, 2221
Thanks Rob. We cross posted. I was pretty sure I was looking at the wrong thing, but didn't know the right terms to search on.

DaveAK
02 January 2011, 2222
I wouldn't know about Radio Trash. ;-)
It's the only thing local. I hate placing an online order for a single small item when the shipping is more than the part.

Edit: Well I fixed the shipping cost problem by ordering a couple more items. :D Thank you both for your help.

DaveAK
03 January 2011, 1046
Ordered last night, and already shipped from digikey. Should have it by Thursday. :D

DaveAK
08 January 2011, 1954
Well, it's all charging at 15A. Now I'm just waiting for the explosion.

Something's not quite right with my charger though. One of the poswer supplies has dropped from 22V down to 16V so I'm only charging at 82V and not 88V like I want to be. Will need to figure that out.

DaveAK
09 January 2011, 1808
Well duh! It's not constant voltage when it's in the constant current phase! One of the four PSUs has the current limiting mod to make sure it doesn't go above 15A, and that's the one where the voltage drops. Eventually the voltage climbed as charging progressed, and then the current started to drop.

Everything seemed to work OKish. Nothing got too hot and the Mini-BMS switched the charger off at end of charge, although I'm not really sure that this was end of charge as I was still drawing 9A. It might be that I need a few charge cycles to bring the cells in to better balance.

One worrying thing that needs investigating is that 30 seconds after it switched charging off, it switched it back on again, steadily charged up to 88.8V (the CV setting), and then switched off. It did this a total of three times I think and then I decided to unplug the power to the charger. It didn't try to switch on again - the solid state relay is power from the BMS and the indicator light didn't come back on. It's supposed to latch off until the ignition is toggled, so either I've got something messed up or the latching circuit is a bit twitchy. Just gives me an excuse to get my MCU based BMS built. :)