One topic I would like to know more about if you have time to explain in terms a layman can understand is the reuse of automotive traction batteries for domestic power storage. There was a scheme in Germany and when I heard about it a few years ago there had been 20,000 installations where batteries deteriorated beyond powering cars were repurposed to store domestic power, charged both by solar and off peak electricity and used in some circumstances to send power back to the grid to help in times of high demand (I can only assume this contribution was negligible) As I was told the cells were re-packed into a unit about the size of a washing machine. A PHEV vehicle I worked on a couple of years ago had a battery pack capable of giving 127 Amps but using it at that level would cause significant deterioration, rapid charging and temperature extremes also did it no good. Domestic use by comparison with controlled temperatures, low loads and charging at a reasonable rate to appropriate levels sounds much kinder but I am a mechanical engineer all I know about electrical things is that they run on smoke and if you let that out putting it back is very expensive. I presume if you were to design a battery for domestic use it would be different from an automotive traction battery, how practical is this reuse?
Hi Ozi, This is an interesting repurposing of batteries. There are a number of factors at play / to be considered. Not sure this will answer everything, but hopefully will help.
As you say, taking a battery from a PHEV and using it in a domestic setting would be sensible in terms of power cycling. The peak and continuous power requirements for the home are quite low. Typically a house in the UK uses ~8kWh of electricity a day (assuming gas heating). Peak loads are probably ~12kW (shower + kettle on together in the morning). Average based on the 8kWh and 24 hours is 0.33kW. Let's assume we use a 12kWh battery pack.
Max discharge is 1C for the house installation, that's quite easy for the old PHEV cells (even at 80% SoH which is the definition used for an aged vehicle pack).
The issues are around the cost of dismantling the original pack and rebuilding it into a new pack for use in a different application. The battery management system will have been set up for use in a vehicle and will expect certain signals/information. You could just use the battery cells/modules and install a new controller. This is ok as long as the new controller can interface with the smaller control boards on the modules. The other issue is the vehicle is likely to have only qualified the cells and controller down to 80% state of health (SoH) and beyond there the cells can behave differently. What I'm saying is the manufacturer of the household packs needs to be talking to the original pack design house to get a number of parameters to ensure that the control system and housing design are safe. All of this is not too bad to do.
Let's assume we have a large solar panel on the roof and we can recharge the panel during the day whilst the house is empty. So, on a normal day we put 8kWh into the pack. There will be losses in the conversion both ways 90% in and out efficiency is a reasonable assumption. For now we can assume the solar panel has more than enough to cover this.
Electricity is ~20p/kWh and so we get £1.60/day worth of free electricity to use at night that we couldn't use from the panel when the sun was shining.
The battery would have probably taken ~3000 complete cycles to drop 20% in state of health (SoH) in the vehicle use. More cycles if it had not been cycled 100%
So, let's assume we can get another 3000 cycles before it reaches 60% SoH. This is the point at which grid storage is assumed to be at it's end of life. Also, at this point our 12kWh pack would have to deliver >100% depth of discharge to give 8kWh. So we would have to curtail the amount of energy delivered to the house. Power delivery should not be an issue as a pack like this should be capable of delivering >5C and hence ~60kW. However, we would have to look at the heat generated in the pack at 12kW and understand how that can work in all ambient conditions. This all assumes the pack is passively cooled and heated (so really will want to operate in a 5°C to 30°C ambient).
In the perfect world this system would save the household £4800. This means we need to get the solar and battery cost below £4800 to make it work financially. If you are charging the pack off the grid and then using the electricity when prices are higher the £/kWh save will be smaller. However, this is not the total story.
Moving forward with electricity prices this equation will get better and this will also give your household electricity security.
Quite a few ramblings here, but hope that helps, there are lots of other reasons for installing this type of system, so please don't take this as gospel, best regards, Nigel
