Small Battery Bargain.....?

[mfj197]

Well, I'm still having a think about it! The issue of connecting differing capacity packs in parallel, that is.

As far as each pack is concerned it is still simply being depleted in isolation. There is no issue with differing voltages across the overall pack as they are hard wired in parallel. So what would the issue be? Both packs would be forced to deplete at the same rate - that is rate of voltage drop, not current rate. They may be delivering different current - a 2.6Ah pack would deliver less current than a 10Ah pack for the same voltage drop - but would that matter? I'm actually coming round to thinking it would be okay to parallel up different capacity packs, as long as you are careful when connecting them together of course! The easiest way to do that is to put a current limiting resistor in between the two packs when you first connect them together.

Michael

 

[JamesW]

Well, I'm still having a think about it! The issue of connecting differing capacity packs in parallel, that is.

As far as each pack is concerned it is still simply being depleted in isolation. There is no issue with differing voltages across the overall pack as they are hard wired in parallel. So what would the issue be? Both packs would be forced to deplete at the same rate - that is rate of voltage drop, not current rate. They may be delivering different current - a 2.6Ah pack would deliver less current than a 10Ah pack for the same voltage drop - but would that matter? I'm actually coming round to thinking it would be okay to parallel up different capacity packs, as long as you are careful when connecting them together of course! The easiest way to do that is to put a current limiting resistor in between the two packs when you first connect them together.

Michael

... so the circuit would look like?

 

[JamesW]

Thanks for the pic jhruk

(don't get worried I'm not about to do that lash up an connect directly to my bike - will have something more robust first)

James

 

[KeithH]

As above, thanks for the pic (and reassurance)
I'll let everyone know if something went bang....after Christmas
K
ps which gauge is the wire?

 

[trex]

you can use 14AWG or 16AWG silicon wires.
http://www.ebay.co.uk/itm/16AWG-Silicon-Wire-Super-flexible-high-temperature-1m-Red-1m-Black-UK-seller-/231295899052
When you connect up the packs with wires, you get a little spark like when you use the jump leads to start a car.
The battery can be put out of action when one of the cells dies.

 

[KeithH]

Thanks trex.
I guess the day a cell dies is the day I open the case!

 

[jhruk]

I used stripped single core 2.5mm mains cable for the battery connection. It needs to be rigid enough to push into the connector.

There should be no spark when you connect the batteries together, only when you connect them to the controller, which you get with any battery.

One of the advantages of a modular battery is that it has an element of redundancy. A single cell failure will only affect that module and you can still carry on using the others. If it was a single battery the whole thing would be out of action.

 

[shemozzle999]

If you want to fit fuses have a look at this - only using this source as it clearly shows the device:

http://www.ebay.co.uk/itm/Medium-ATM-Fuse-TAP-Add-on-Dual-Circuit-Adapter-Auto-CAR-Holder-7A-10A-15A-20A-/251255839557?pt=LH_DefaultDomain_3&hash=item3a8003db45

While not specifically made for parallel connections it can be adapted.

The lower blade is commoned to the two fuse holders and will go to the controller, the upper blade and the flying lead would go the the battery positives.

Select a pair of fuses rated at just over half the maximum current required i.e. For an 18a controller select 2 x 10a fuse.

 

[PeteB1951]

I got a couple of the 4.0 Ah ones and a charger off eBay a few months ago, I'm intending to use them to power a Brompton nano kit I got second-hand, and the clearance price has prompted me to get another two. So I now have four batteries and three chargers for a total cost of less than £200. Making a battery connector has been what's holding me up, but now I have a charger to cannibalise and will still have a spare. I'll only make a connection for a single battery, if I need more range I'll carry a charged spare (or two) and swap them over - the batteries click very simply into and out of the connector.

I've been inside both the battery and the charger - both look quite well put together with decent components and construction. There are 20 Samsung INR18650-20R cells, which go for £7.20 each on Amazon to power e-ciggies. Unfortunately the charger only has three spade connections - two +ve ones, inner and outer, and an inner -ve one, but there isn't a spade for the outer -ve connector on the battery which is the important one. However it does look like it will be possible to move the inner negative across if I cut a new slot for it in the plastic casing.

Here's a link to a spec sheet and test results for the cells http://budgetlightforum.com/node/19898
It states that they are good for up to 22A discharge, which should be OK for most ebike use.

Pictures of the battery and charger attached:

 

[Proboscis]

Can you tell us what the markings on the big chip inside the battery are? I can't quite read the numbers.

And any chance you could take a picture of the board on the edge of the battery (think that's the LED display board)?

Looks like there's I2C so it's possible to get charge state out of the charge controller programmatically - eg to do some kind of managed switchover. I'm also curious what's under the heatsink in the middle - whether the battery can switch off its own output when it's getting low.

 

[Deleted member 4366]

Looks like there's I2C so it's possible to get charge state out of the charge controller programmatically - eg to do some kind of managed switchover. I'm also curious what's under the heatsink in the middle - whether the battery can switch off its own output when it's getting low.

Of course it can switch itself off when the charge is low. That's the main purpose of the board. Under the heatsink are two MOSFETS, which do the switching. The charge MOSFETs, which switch off the charging are below them in the photo next to the two solid state shunts, which tell the microprocessor how much current is flowing. You could solder another shunt on top of one if you want to increase the current a bit.A 010 would allow 20% more current..

If you don't want to take too much current (250w system), you could unsolder one pack from its BMS and wire the connections directly in parallel to the BMS connections on the other pack, so you have a single 8ah battery with one BMS.

The board on the side looks like a standard LED charge level indicator. It just takes a 36v output (charge level) from the main board an it works independently. You can take it off and stick it anywhere on your bike, but a voltmeter would be better.

 

[paulhipwood]

We have bought two of these batteries, purchased on a Wednesday from B&Q, you can get 10% extra off, if you are 60 or over. Makes them more of a bargain.

I have a plan to use a 5 mm plastic kitchen chopping board to mount them on.
Will use 5 layers and arrange them so that it has a pair of steps either side for the battery to slide in.
I will use strips of steel let into the middle large layer to trap them somehow.
So the batteries will be mounted with the connectors facing each other.
When I have done this I will post a picture.

regards
paul

 

[jhruk]

Here's my first rather crude attempt at a single connector using some scraps of wood. It works ok if the battery is in a bag but I wouldn't want to mount it straight on the bike with this. I'm still contemplating the mark II version.

Look forward to seeing your dual 'chopping board' model – I rather like the sound of that.

 

[Proboscis]

Of course it can switch itself off when the charge is low. That's the main purpose of the board. Under the heatsink are two MOSFETS, which do the switching. The charge MOSFETs, which switch off the charging are below them in the photo next to the two solid state shunts, which tell the microprocessor how much current is flowing. You could solder another shunt on top of one if you want to increase the current a bit.A 010 would allow 20% more current..

Apologies if that was obvious - my NiMH pack doesn't have such control, it's switched in the bike.

Anyway, some staring at the picture suggests it's an O2Micro BMU chip - from the O2Micro range the only one in this package would suggest it's an OZ8940. That could be programmable using the software from BMSBattery (I assume the 'you have to buy our $200 USB dongle' is a sales ploy - the O2Micro software supports parallel ports without needing drivers, for instance).

 

[trex]

you can perhaps use a cheapo arduino shield to connect your PC to the I2C instead of using the O2Micro kit to interface the OZ890.

 

[PeteB1951]

Apologies if that was obvious - my NiMH pack doesn't have such control, it's switched in the bike.

Anyway, some staring at the picture suggests it's an O2Micro BMU chip - from the O2Micro range the only one in this package would suggest it's an OZ8940. That could be programmable using the software from BMSBattery (I assume the 'you have to buy our $200 USB dongle' is a sales ploy - the O2Micro software supports parallel ports without needing drivers, for instance).

I don't want to open the battery again (there are some plastic clips inside which won't stand much unclipping and reclipping), but looking at my original pic rather than the compressed version I uploaded, the chip does seem to be marked OZ8940.

Also here's a shot of the battery indicator which I didn't upload earlier:

 

[robbyheansen]

nice link!

 

[craiggor]

If fast charging is bad for battery life.would there be a way of slowing this charger down.it is rated as 3.6 - 3.7 a.Have you all seen the makita ebike that runs on one or two 18v batteries.it's on you tube.and monster garage put 384 Milwaukee drill battery's in the boot of a 1962 Chevy.

 

[patpatbut]

Custom made connection. So far so good

 

[craiggor]

looks like the price has gone up.