readiescards, 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
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.
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.