Super Capacitor Car Battery Project
There are plenty of resources on the net about how to use Super Capacitors as car starter batteries. There are a number of kits and pre-built items available. For a price.
Super Capacitors as an energy store have advantages and disadvantages. There are plenty of online discussions for comprehensive info about that. I will describe a project that leverages the advantages while minimising the disadvantages.
Why:
I have a car in New Zealand that gets used a couple of times a year (while I am visiting family). It is used maybe 6 weeks in total each year. Leaving them parked up for 5 months or more at a time causes problems for traditional lead/acid batteries leading to them simply give up. Every few years I have to replace the battery which makes those visits a bit more expensive. What is needed is an energy store that does not break down when not in regular use. The replacement also needs to be less expensive than a couple of lead acid batteries (that is, must remain cost effective over time).Pro:
- Can store sufficient energy to start a car (in suitable configuration)
- Can be charged/discharged with much greater frequency than a lead acid battery
- Can be fully discharged without damage
- Much lighter than Lead/Acid battery
Cons:
- Tend to be low voltage devices
- Caution is needed to prevent accidental discharge damage to attached electrical devices
- Drain in Volts quicker than Amps. Yes I know that's not real terminology, but that is how I think of the energy loss in use. Voltage loss is important because most cars have small energy using devices even when the car is not being driven.
The biggest problem is the energy drain when the car was not in use. One of the ways to overcome slow drain is the addition of Lithium Ion batteries in parallel to the capacitor pack. While this seems to be a nice idea, the battery packs are expensive for the voltage and energy required. Like most battery packs they do not like to be fully drained (something likely to happen when left for extended periods of time).
LiFePo batteries will handle mass discharge, slow recharge, and do not easily lose power when not in use. This introduces an alternative - a battery pack that can be utilised to 'top up' the super capacitors when required, charged when the car is running, but otherwise can be left unattended.
Another option is to somehow maintain Super Capacitor voltage when not in use. While there are lots of options available for this, the easiest is small solar 'battery top up' panels run through a buck-boost controller.
So, the base proposal is:
- Super Capacitor pack for day-to-day use
- Battery backup for start-up boost if required
- Solar trickle charge to maintain voltage (where possible)
It is useful to make a couple of additions:
- Some way of connecting the boost battery without the hassle of getting out and making connections
- some way of automating boost battery recharge (battery charge control)
- adding a dedicated 5v system for USB devices (especially useful for older cars like mine)
All of that while keeping the cost down!
Suitable Super Capacitors
Most Super Capacitors have a voltage capacity around 2.5 to 2.7 volts. In order to get the required 12v we need at least 6 in series. Some designers insist that balance circuitry is required in order to ensure the capacitor integrity. My understanding is that these balance circuit result in a small amount of energy bleed when not in use which is one of the things we are trying to avoid. In practise it is agreed that balance circuitry is not required provided that rated capacitor voltage is not exceeded, most suggesting that 80-90 percent of capacity provides a safe margin.Most of the systems I have seen online are for smaller vehicles and are a series of 350 Farad capacitors (some bigger, some smaller). Small car engines require less energy to start than larger engines. This project is for my Nissan 300zx, a 3 litre V6. This does need a reasonable amount of oomph to get going.
So, I got lucky and scored a dozen 600 Farad @ 2.7 volts for reasonable money (as at July 2016 I can get these for about $78USD for a bundle of 6). They are Japanese made, have bolt type connections, and seem to be quite sturdy in construction.
I started the experiment by using 6 Super Capacitors with a nearly dead battery. That worked rather well - the battery held enough charge to keep all the other car electrics running for a day or two, while the caps start the car no probs. That stayed in place for a fortnight use.
Next step was to remove the battery altogether However... It seems there are a couple of tricks not really documented anywhere. The one I found out about fairly quickly is that they will not hold charge well until they have been cycled a few times. After a few uses it settled, but because of fairly high drain I added a second string of Super Capacitors just to be sure it was robust.
My first run was at night with lights etc and the caps drained after half an hour of non use and I was stranded :/ oops. This is when I decided a booster pack would be a good idea :) Mini size LiFePo booster packs are finally coming down in price. They come in a variety of capacities, ranging from 18 Amps (for small cars) upwards. I managed to obtain a 38 Amp (300peak) booster for $78USD. The addition of a dash mounted push switch that operates a 60 Amp relay makes it possible to recharge the capacitors simply.
Recharging the boost pack is handled by way of a small timer. This receives a signal from the ignition 'on' position. The timer then counts 2 minutes before switching its relay to allow current into the charge socket of the boost pack. It remains charging for 999 seconds (about 16 minutes 40 seconds) before turning off. This process is repeated each time the car is started.
In order to make adding devices requiring small amounts of current easy, I added a couple of WAGO connectors (one Positive, one Negative). These lever actuated connectors will handle up to 30 Amps of current. A 10 amp fuse is included on the positive line to the Super Capacitor pack.
There is also a 6 point connector, 3 are currently in use leaving three for later use.
The end result is significantly lighter than the lead acid battery it replaces. The lack of fumes allows this whole assembly to be inside the car cabin. It is mounted onto an existing floor cover and a new cover is put over it for protection. An earth cable is connected to an existing internal mount point. The positive cable runs to a threaded rod that goes through the firewall via an isolating grommet. On the other side of the firewall there is another cable going to the original terminal point. This minimises the need for any factory wiring alteration :)
Update:
When the car was being inspected by an engineer (certification of other work) he noticed the extra wiring and went searching. He found the amended battery system and used words like 'you are not installing LiFePo batteries as a permanent part of this car' and removed the whole assembly. While it is true the battery is attached, it is not always connected (a small but important thing to overlook). He managed to get pretty much every other thing wrong in his inspection so I guess I should not be too surprised. And his messing me about has allowed the registration and licensing to lapse, which is costing nearly $1000NZ to rectify.I was mostly not favourably impressed because in the process he damaged both the wood covers custom made by number one son. I have yet to replace the original that he also damaged. grrrr.
So right now the car has a standard (dead) battery in place and has to be boost started every time my mechanics need to move the car.
At some point (after the car is legal again) I will rig up a small 12v battery just to get the car started and once it is running pull the battery out. I can leave that plugged in to a smart charger at a friends' place and forget about it. Not ideal but if it works for the mean time then shrug.