Author Archives: Greening Me

Transportation update

It’s approaching sixteen years since I bought my first electric vehicle.

Things have moved on enormously from my first electric vehicle – a REVA G-Wiz. The G-Wiz is a small vehicle similar in size to a Smart car, but strictly was not even a car at all but rather a quadricycle. As a quadricycle it is limited by law in both weight and power resulting in a tiny underpowered vehicle compared to even a small car, however that was all that was available at the time. It was good for 30 to 40 miles according to time of year and speed and maxed out at around 50 miles per hour although 30-40 miles per hour was more achievable.

2007 REVA G-Wiz
2014 Vauxhall Ampera

In the summer of 2014 I bought a new Vauxhall Ampera. I’d initially seen the Ampera at a motor show and dismissed it as neither a full electric vehicle or a plug-in hybrid. However a few years later and now married with a young child I was looking for something bigger. Now I saw it as being more capable than the G-Wiz in size, range, and safety with the bonus of having an engine for the occasional longer trip. I’ve now owned it for over eight years.

The latest addition to the household is a 2020 Ford Kuga PHEV. This is similar to the Ampera in many respects with some variations.

2020 Ford Kuga PHEV
AttRiutereva g-wizVAUXHALL Ampera Ford kuga PHEV
Usable battery size10 kWh10 kWh10 kWh
Vehicle typeBEVPHEV/E-REVPHEV
Range30-40 miles30-40 miles30-40 miles
Seats2+2 seats4 seats5 seats
ConnectedNoNo (at least in Europe)Yes
Electrical capabilityLow (heavily compro-mised versus ICE)High (Same as ICE)Medium (Poorer than ICE but keeps up with traffic)
Electrical inlet connector3-pin caravan5-pin BS EN 62196 Type 1 / J1772 7-pin BS EN 62196 Type 2
ICE capabilityn/a (BEV)Conventional 1.4L Atkinson 2.5L (for economy)
Transmission Simple reduction between electric motor and wheels. No ICE or variable transmission ratio.Similar to Kuga but has additional clutches and brakes allowing transmission to either work as powersplit or separate into a series hybrid with a generator set connected only electrically to the motor and wheels.Powersplit – combines two electrical machines with torque trans-mission from ICE to wheels via both electrical and mechanical means. Lacks the ability to split the transmission into a separate generator and motor set.
Comparing my plug-in vehicles
Powersplit transmission

Both Ampera and Kuga use epicyclic or planetary transmissions to create a variable ratio between engine and wheels. Both gear together three shafts such that the speed and torque on any two shafts determines the speed and torque on the third shaft. Both have the wheels connected to one shaft, the engine to the second, and an electrical machine to the third. However Ampera and Kuga differ in that Kuga places the second electrical machine on the same shaft as the wheels, while the Ampera places the second electrical machine on the same shaft as the engine (which helps when splitting the transmission to create a series hybrid).

I’d differentiate the Ampera from the Kuga as describing the Ampera as a short range electric vehicle with an ICE for occasional longer trips, while the Kuga is more of an enhanced hybrid optimised more for those longer trips with a higher efficiency Atkinson engine. The Ampera on the other hand has greater electrical capability (uncompromised performance electric versus ICE) with greater transmission sophistication having the ability to switch between being a series hybrid or a powersplit.

Battery Economics

Having recently spend around £600 replacing the internal batteries in my Powervault G200, I thought I’d look at the economics of that decision.

Powervault G200 storage

Originally I bought the Powervault to store the surplus electricity from my solar panels and then use that electricity later instead of buying electricity. I don’t receive any revenue for exporting electricity so to fill my battery from my solar surplus costs me nothing. I currently pay 35 p/kWh for daytime electricity. So, if I fill my battery from my solar surplus then I save 4 kWh (the capacity of the battery) x 35 p/kWh = £1.40 per day.

More recently I also charge the battery on cheap overnight electricity when there won’t be enough solar to charge the system fully.

I only get four hours of cheap power which isn’t enough to fully charge the battery. The battery charges at 0.8 kW so in four hours would use 4 hours x 0.8 kW = 3.2 kWh which at 7.5 p/kWh costs 24 pence. The system has a cycle efficiency of 80% so 3.2 kWh in gives 3.2 kWh x 0.8 = 2.56 kWh out. 2.56 kWh energy out is worth 2.56 kWh x 35 p/kWh = 89.6 pence. Having spend 24 pence to save 89.6 pence then I’ve made net savings of 65.6 pence per day.

G200 User Portal

A day like that shown in the portal with 4.75 kWh in savings is a bonus in terms of annual savings although it may accelerate the deterioration of the battery as it’s been double-dipped during the day with both overnight and solar charging.

If I assume that half the time the system is filled by solar and half from the grid then my annual savings are 0.5 x (£1.40 + £0.656) x 365 = £375.22. Having spent £600 on the new batteries then my payback time is £600 / £375.22/year = 1.6 years. The previous batteries lasted for 6 years although the new batteries were relatively cheap and might not be expected to last for so long. However even if the new batteries only have half the life of the prior ones then that still seems like an attractive investment.

My total investment in the battery storage is now £2600 – £2000 to buy it six years ago plus the £600 just spent on new batteries. That’s a payback of about seven years with expectation that the new batteries will extend the life of the system to nine years.

Reviving the battery

Powervault G200

My Powervault battery is now six years old. Initially I used it only to store surplus electricity from my solar panels but more recently have also managed it from my HEMS to reduce my energy costs by charging it when electricity is cheapest to offset consumption when electricity is most expensive. However it’s become clear in recent weeks that the working capacity of the battery had significantly declined.

Powervault portal for G200 before battery replacement

My Powervault is designed to have a reasonable life – its 7.92 kWh internal batteries (6 batteries x 12 Volts x 110 Ah) are limited to 4 kWh to maximise the number of charging cycles. However after 6 years working capacity had dropped to less than half of that and I decided that the time had come for replacement. Declining battery capacity is illustrated above with the blue line showing battery charging between 00:30 and 04:30. If the battery had continued at constant power (about 750 Watts) then it would have charged 3 kWh (4 hours x 750 Watts / 1000 Watts/kW). However here the battery has ceased to draw 750 Watts from around 02:00 and thereafter power consumption has significantly declined – this may be about 1.75 kWh rather than 3 kWh.

These figures (1.75 and 3 kWh) are also energy into the battery – but you don’t get out everything that you put in. With a quoted round trip efficiency of 80% then 1.75 and 3 kWh in would deliver 1.4 and 2.4 kWh output versus the original capacity of 4 kWh.

I reached out to both my installer and Powervault themselves but neither got back to me with any proposal to investigate or replace the batteries so I resolved to do it myself.

Powervault G200 internals

Initially I did a little investigation by removing the top and side panel of the Powervault (after turning off the power). Inside I found six twelve Volt one hundred and ten Amp hour batteries arranged over three shelves on plug and socket connections – so fairly straightforward to replace. A search of eBay for deep cycle batteries of similar capacity indicated prices range from around one hundred to around three hundred pounds per battery (six required). I eventually decided to go for the cheapest as I wasn’t entirely sure of success and I don’t know for how long my six year old system will be supported even if battery replacement was successful.

6 x Numax CXV31MF Batteries 105Ah

The Numax batteries are sold as deep cycle batteries ideally suited for solar installations.

Powervault G200 installation.


The process for installation of new batteries consists of unplugging and removing all the batteries from top to bottom, and then installing new batteries from bottom to top – two on each of the three shelves. I found it necessary to undo the fixing screws on the back of one of the shelves in order to flex it upwards to create enough clearance to get the batteries on the level below out.

Powervault portal for G200 after battery replacement

It’s only been just over a week now, and it’s fairly dull weather at the moment, but it does seem that the replacement has been successful. The dashboard above for example shows both that the system has charged at full power for 4 hours overnight (3 kWh in) and also that 4.75 kWh has been saved (versus 4 kWh working capacity) as to a degree there’s some double-dipping going on with the two partial charges of more than half capacity (one overnight on cheap mains power and one during the day on surplus solar power).

Time will tell what the longevity of these cheap batteries is.