How to replace electric car batteries

electric car batteries

Replacing electric car batteries

Choosing electric car batteries is important but depends a lot on the use they will be put to. Yes, of course, they will be used for an electric car, but there are many considerations before settling on a particular type. The photo below is the original setup with voltages marked on the individual batteries as they deteriorated.

So how do we go about choosing electric car batteries?

  • The type of car
  • The speed of the car
  • The use it will be put to
  • The weight of the car
  • The motor to be used
  • Cost

Let’s tackle each of these in turn. The type of car is determined by the use it will be put to. Is it to be a sports type of car? An SUV? A town car? A truck or an intercity car?

An SUV despite its name is not a sports car. What sport do you do in your SUV? It’s a Utility Vehicle. As such the best batteries are either LiOn or if individual 12 volt batteries, LiFe are good.

A high-speed car needs a big motor so for heavy current stay with LiOn.

For running around town, probably the best would be to go with LiFe or AGM. I’m not in favour of gel batteries any longer due to their relatively short life and weight.

Motor? As mentioned earlier, the bigger the motor, the more power it will take, so for small motors, again, LiFe or AGM.

The cost of the batteries is of course related to their power output. I don’t mean voltage, I mean current. LiOn is the most expensive, followed by LiFe, then gel, and finally AGM. Don’t consider ordinary “flooded” batteries as they run out of steam relatively quickly and the liquid acid is more dangerous to yourself and the car.

When you buy an electric car the question of which type of battery to use has already been thought out by the manufacturer, but things change quickly in the automotive world, particularly in the EV market where the most rapid development is taking place. Electric car batteries which were the best last year may not be the best this year. It may also be that the reason you bought the car has changed, and a different type of battery may be more efficient for your new use.

Changing the use of the car

You may wonder what we mean by changing the use, but let’s say you bought an EV for commuting to work every day but you have retired, or you have bought a new one and passed the previous one to another family member who has a different purpose for the car. When the batteries need changing, are the present types still the best choice for the new use?

Gel-type electric car batteries

My own car was supplied with gel batteries as standard, but I have found after 4 years that as they deteriorated, they don’t all do so at the same rate. One of the 12-volt gel batteries would no longer charge to 13 volts, which is a nominal voltage for a fully charged battery. Another quickly followed and gave only 10 volts after charging. So what does this mean?

gel is finished now it's lithium iron batteries

In my case, My use of the car was as a town run-about. The manufacturer’s gel battery specifications were 12 volts, 100Ah x 6 giving a total of 72 volts to the motor. The weight of these batteries was 38 kilos each or 38 x 6 – a total weight of 228 kilos. For around-town use, I went one better. AGM batteries are lighter for the same output of 12 volts at !00Ah and cheaper to boot. Wonderfully, they also last a couple of years more than gel batteries. LiFe PO4 batteries would of course have been lighter, more efficient and unfortunately, more expensive.

Physical size of electric car batteries

Don’t forget that when replacing the batteries with a type which was not originally supplied with the car, to account for physical size. The batteries are fitted into a container or frame which is usually designed for the original size units. Even amongst batteries of the same voltage and current rating, different manufacturers may provide different-sized batteries, so make sure they will fit their carrier before buying.

Check the terminals

Another point to take into consideration is that the manufacturers of certain electric car batteries, may have made the battery with a particular use in mind. You may find the positive and negative terminals transposed, or again, you may find that the batteries have round post connections rather than 13mm bolts or vice-versa. All these points should be noted as terminal connections may need to be changed in the process.

I could have replaced them with LiFe (lithium-iron) batteries which would have lasted longer and been still lighter, but the expense would have been greater than the gel batteries. AGMs are perfect for my use but they needed to be balanced. Read on for more on this aspect.

As a battery deteriorates with age, its voltage drops off a little, but worse than this, the amount of charge it will hold tends to drop off dramatically. When a battery shows a voltage far less than its nominal voltage, for example, 10 volts instead of 12 -13 volts, this usually indicates more than deterioration. Often it is an indication that one or more of the individual 2-volt cells comprising the 12-volt battery, has developed an internal fault. There is no way to rectify the situation and requires all the electric car batteries to be replaced.

Can I replace just one or two electric car batteries?

If a battery fault develops amongst a set of car batteries, (often 5 or 6 batteries are in use here), it is not a good idea to replace the faulty one alone unless the whole set of batteries is a very recent purchase. Let’s look at a situation where a 3-year-old set of 6 batteries makes up the required 72 volts for a particular car. As each of the other batteries is already 3 years old, they will no longer be able to hold the same voltage or current as a new battery.

This is where we need to understand a bit more about batteries.

What happens, in this case, is that a new replacement of one battery out of 6 will not receive a full charge due to the charger cutting out after charging the others to their reduced capacity. The only viable option is to replace all the batteries at the same time. Then we are faced with what is known as battery balancing.

Battery balancing is the process of equalizing the voltage and current capabilities of all 6 batteries, or “equalizing” them. This is a process which is done by a couple of methods. Each battery can be fully charged individually by use of a 12-volt charger. Now that we know they are all fully charged, they can be replaced in the car. This is not the only method, however. Another method which balances them more accurately is before the batteries are fitted into the car, connect all the + terminals together, and all the – terminals together. As they are now connected in parallel, the voltage will be 12 volts.

Connect the 12-volt charger to the + and – terminals and charge the batteries on low power charge for at least 24 hours. Remove the charger, remove the connecting wires between the batteries, and check each battery with a test meter. Each of the batteries should show an identical voltage. The batteries are now ready to be replaced in the car.

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