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Dumb Question: How Many AA Batteries Do I Need To Get 48V And 5Kw


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#1 readiescards

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Posted 20 January 2016 - 01:57 PM

I'm rather embarrassed to ask this question given I've a degree in Electrical & Electronic Engineering (from a long time ago), but here goes:

The Tesla car battery appears to be made of many AA batteries, if I want to make my own 48V of say 5kW how many 1.2v 3000mAh do I need?

48 / 1.2 = 40 batteries in series.

And this is where I'm confused.

Do 40 of 3000 mAh 1.2v batteries in series at 48v provide 144 Watts?

And therefore for 5kW I would require: 40 * 5000 / 144 = 1388 batteries

Correct?

1388 AA rechargeable batteries at £0.50 about £700

Edited by readiescards, 20 January 2016 - 02:43 PM.


#2 SteamyTea

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Posted 20 January 2016 - 02:04 PM

Don't it depend on how much you discharge them.
I am sure there is a voltage/amps discharge chart for different battery types.

#3 jsharris

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Posted 20 January 2016 - 02:45 PM

View Postreadiescards, on 20 January 2016 - 01:57 PM, said:

I'm rather embarrassed to ask this question given I've a degree in Electrical & Electronic Engineering (from a long time ago), but here goes:

The Tesla car battery appears to be made of many AA batteries, if I want to make my own 48V of say 1Ah how many 1.2v 3000mAh do I need?

48 / 1.2 = 40 batteries in series.

And this is where I'm confused.

Do 40 of 3000 mAh 1.2v batteries in series at 48v provide 144 Watts?

And therefore for 5kW I would require: 40 * 5000 / 144 = 1388 batteries

Correct?

1388 AA rechargeable batteries at £0.50 about £700

The Tesla doesn't use AA cells, it uses 18650 laptop cells, which are lithium ion chemistry, with a nominal terminal voltage per cell of 3.7 V and a fully charged voltage of 4.2 V (although charging them to about 4.15 V extends their life a fair bit).

To get a 48 V nominal voltage battery, you would need to connect 48 / 3.7 = 13 cells in series ***. If the capacity of a single cell was 3000 mAh (3 Ah, which is typical for a reasonably good 18650 cell) then the capacity of the whole battery would be the same, 3 Ah. In practice the battery pack voltage would very between a minimum of about 46.8 V with the cells discharged to 3.6 V per cell (about as low as you want to go for long life) and a maximum of about 53.4 V when fully charged to 4.15 V per cell.

In terms of energy, such a pack would store around 144 Wh, so enough to run a 10 W load for very roughly 14.4 hours. However, for long life it's best to only charge the battery pack to about 95% of full capacity and only discharge it to around 50% of full capacity (this is pretty much what all the electric cars do). This reduces the usable energy from around 144 Wh down to around 65 Wh.

*** WARNING!!!

You cannot just connect lithium ion cells in series and charge and discharge them without a battery management system. This battery management system measures the voltage of every cell in the pack and prevents any cell from exceeding 4.2 V on charge (or a lower voltage of 4.1 to 4.15 V per cell). If any single cell in the pack is allowed to exceed 4.2 V during charge then at best it will expand and burst, at worst it will explode and catch fire.

Similarly, when discharging the battery into a load, the battery management system will switch off the load if any single cell in the pack drops below the preset "kill voltage". For safety this is usually around 3.4 V for the type of lithium cells used in the Tesla, but to prevent over discharge shortening the life of the battery pack the cut-off voltage is usually somewhere around 3.6 to 3.7 V off-load voltage (on load the voltage will drop due to cell internal resistance).

I've designed and built a fair few battery management systems for different lithium chemistry cells, and it is not straightforward. You have to have constant voltage charge current shunts across each cell* that will turn off the charge to that cell at the full charge voltage yet still allow charging current through to the rest of the cells in the battery pack. You also have to have a way to turn off the load safely - in an electric car this is done in stages often, warning the driver that the battery is running low and reducing the available power before the final battery cut off point.

* By cell I also mean any group of cells in a lithium battery connected in parallel, to increase the capacity of the cell group whilst maintaining a single cell terminal voltage.

As it happens, I've just come in from putting up another 10 W solar powered, PIR triggered, outdoor floodlight, that uses three 18650 cells (as used in the Tesla) in series plus a solar panel. I cheated and purchased a 3 cell battery management module from ebay, as it wasn't worth the cost of making one.

BTW, if you wanted to build a 5 kWh nominal battery pack, with a nominal 48 V terminal voltage, using 3 Ah 18650 cells, then you would need around 455 cells, wired as 13 series connected banks of 35 parallel cells per bank, together with a 13 channel BMS (battery management system). The BMS would need to be rated at the maximum charge current you envisage.

Edited by jsharris, 20 January 2016 - 03:32 PM.


#4 ferdinand

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Posted 20 January 2016 - 07:53 PM

I think you may get a good answer to this on the e-bike forum http://www.pedelecs.co.uk/, as they do use AA batteries, albeit usually in battery packs a little smaller than that.

Ferdinand

Edited by ferdinand, 20 January 2016 - 07:54 PM.


#5 jsharris

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Posted 20 January 2016 - 09:23 PM

The snag with the pedelecs forum is that there is a dearth of in-depth knowledge, though. There are a couple of outspoken individuals who give really, seriously, bad advice on home-made stuff, and frankly I don't know how one or two of them haven't had serious accidents before now. It's also commercial, and effectively run by commercial ebike vendors, so anything that smacks of going against their commercial advertisers interests will get moderated, heavily (I speak from personal experience!). There is a very good electric vehicle forum, endless sphere, that does have some real expertise amongst it's members, though, and they will never censor or moderate any post unless it's offensive or advertising a product.

#6 ferdinand

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Posted 20 January 2016 - 10:03 PM

Fair comment, J.

#7 readiescards

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Posted 21 January 2016 - 07:18 AM

Thanks Jeremy for the detail explanation and warning! I'd be interesting in knowing which BMS you used for your solar PIR light as well as being useful for my site it might be a good introducing for me to learn the ins and outs of battery management

#8 jsharris

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Posted 21 January 2016 - 08:30 AM

THis is the BMS I used: http://www.ebay.co.u...cIAAOSwLVZV6dOZ although I bought mine direct from China for a bit less than this UK seller is asking for them.

This is the circuit diagram:

Attached File  solar light diagram.pdf   23.88K   6 downloads

This is one of the finished lights on the front of the house:

Attached File  10W LED Outside light - fitted.JPG   31.88K   2 downloads

And this is both the front and side lights working, taken last night:

Attached File  Lights working.JPG   16.5K   2 downloads

The solar panels used were cheap ones from China, around 2 to 4 W at about 18V maximum. I didn't bother with any form of charge regulation or whatever, because the maximum current from the panels will be around 250mA at most, which is way below the max charge current of the cells. The BMS module will just turn off when the cells are up to fully charged voltage, and prevent over-charge. When this happens the solar panel will be open circuit, in effect, but the open circuit voltage is well below the safe maximum input voltage for the BMS module. The Shottky diode is there to prevent the battery pack from draining back into the solar panel at night. I used cheap 2200 mAh 18650 cells, but did shop around to get decent, rather than fake, ones. There are literally millions of fakes on ebay, so it's best to try and get some brand name ones from a reputable supplier. The PIR module is one of a few I bought some time ago from ebay, with a built in MOSFET power switch. I've just had a look and these don't seem to be around now, but there are other units about that will do the job, I'm sure.

The energy budget for this light is quite conservative. It's set to stay on for 3 mins when triggered, and I doubt it will be triggered more than a handful of times a night, say 15 minutes total on time, which is around 2.5 Wh per 24 hours. The PIR module drains 50 µA all the time, so drains a further 0.0144 Wh per 24 hours, which can really be ignored, as it's so low. The battery pack only uses less than 1/4 of its capacity to run the lights, so should last a long time. The solar panel has to provide around 2.5 Wh each day to recharge the battery pack fully for the worst case of a night where the light is activated five times, and a 2 W rated panel should be able to deliver this sort of energy level over daylight hours on even an overcast day in winter, as it only needs to provide about 0.3 W during an 8 hour winter day. The battery capacity is great enough for the light to go for two or three days with no charge at all, so even if it doesn't get fully charged in a single gloomy day, the chances are it will on following days. It's also unlikely to be switched on five times a night, so these calculations really are worst case.

The batteries, BMS module and PIR module and switch all fit into a small watertight plastic box that is 65mm wide, 116mm tall and 40mm deep, with the solar panel glued on to the top. I fitted an alloy bracket at the rear to fix it to the wall at the right angle and the solar panel provides a bit of protection and shade for the stuff in the box. The 12 V LED floodlight is separately mounted, so it can be angled to where the light is needed. I'm quite pleased with the result, as these 10 W LED floodlights give ample light for getting out of the car to the front door and seeing to get the key in. The ones around the side illuminate the path from the back door to what will be the wheelie bin storage area, next to the meter box fence.