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Practical aspects of the lightweight Li-Ion batteries

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First, wanted to tell a little story that happened to me a few weeks back. Scroll to the bottom of this post for TL;DR, I won't blame you ;)

Earlier this year bought a new Exige specced with a lightweight Li-Ion battery. As a weight saving NCO (compared to £1350 in previous models) it seemed attractive enough and only made sense to have it fitted instead of a regular battery. These batteries actually give a pretty solid output for the size. However, as I found out later, the problem with these is that they have a lot less capacity than standard acid batteries. The owner's manual supplement states that, depending on various factors, they can go from fully charged to flat in about 10 days. The reasons for that are some electronics which continue to drain the battery even when the car is off on the one hand and self-discharge (aka ambient discharge) on the other.

So, with that in mind, a few weeks ago I went on a trip for seven days hoping it wouldn't be enough for the battery to go completely flat. Obviously that was not the case, otherwise, I wouldn't have had much to write about - the car was completely dead when I came back! Immobiliser off, turning the key does nothing - dead. Luckily enough and somewhat anticipating the turn of events, I hadn't armed the alarm and instead locked the car mechanically before I left. Have to mention, if your battery goes flat and you'd locked your car with the fob you're extra eff'ed, as the only way to get inside and open the boot is through smashing a window (words of a Lotus engineer). So at least getting inside was an easy job; figuring out what to do next wasn't.

First thought: these batteries come with a protection device that cuts out at low battery voltage to protect from dropping the charge too low. The device has a reset button that can give you a little power if there's any left. Press. Press again. Nope.

Next: jump starting. Forget about it, that's a big NO, you cannot jump-start a Li-Ion. Next option: trickle charge using a battery conditioner (mine's Optimate Lithium). See, I keep the car in an underground car park which is a very secure place but has exactly zero electric sockets anywhere on the premises. The conditioner is AC-powered. Baws.

Out of ideas at this point, opening my contacts on the phone and calling Phil from Bell&Colvill (huge shout out,  many thanks to Phil for guiding me through!). He suggests calling Lotus Assist. LOL, I didn't even realise I had 2-year breakdown cover. OK, so I give them a call, they put it through to AA and soon tell me that a rescue vehicle is on its way. Not even an hour later, a fella in a patrol minivan arrives and I take him to the car park. I give Phil another call and let him speak to the rescuing guy to figure something out. Surely AA would have the necessary equipment to bring a dead Lotus back to life. Maybe a generator? No? No. 

Day 2: calling AA again, this time asking for a towing vehicle. They enquire about the car park entrance dimensions, I measure and tell them, we're good! Slightly over an hour later, it arrives. A truck, a big truck! No idea why they asked about the dimensions because that thing was never gonna fit :) The gentleman who came with the truck suggested we push the car out of the car park, to which I said "no way! There's like a 10° lengthy-ish uphill slope leading to the exit gate. Lotus make light cars but not that light!". In hindsight, we could have done it. Hell, they guy could have done it even without my help - that dude was mighty strong! In any case, I had my second vehicle parked nearby so I suggested we use that to tow the Lotus out of the car park. Exige Series 3 (all V6 models) have a tow bar hidden behind the front number plate and the radiator grille. Unscrew a bunch of screws, take both off and that's how you get to it - a little bit of work but nothing too hard. My other car had a tow eye and a little niche at the back where you screw it in. The rest was an easy job of towing the Lotus out, made even easier by the unassisted steering.

Once the Lotus was loaded on the truck I kindly asked the driver to drop it off near my place so I could stretch a 50m extension reel, connect the Optimate to the mains and give the car a good charge. That's how the story ends. Some lessons learned:

  • Unlike with most other cars, the Lotus driver's manual, as well as any supplements, are required reading
  • New Lotus cars come with a free 2-year breakdown cover, at least in the UK
  • Li-Ion batteries can be a real PITA. No jump starting. The only way to bring it back to life once flat is by trickle charging (AC power required)
  • Can't rely on recharging by driving unless you're using it every day, best to give it a full charge once a week
  • Never close the car with the key fob if leaving the car for a lengthy period of time (> week)
  • Wish Lotus had made it easier to remove the battery so you could charge it at home if needed
  • In general, very pleased with AA, great and reasonably fast service!


  • A bit surprised there are no DC-powered trickle charges for Li-Ion car batteries. Logically, Li-Ion batteries (e.g. in mobile phones) can be charged with portable sources of energy (e.g. power banks) - why not car batteries then?
  • I remember reading somewhere that someone had a setup using a secondary battery (slave) connected to the main one (master) for longer life while parked. Anyone done / heard of anything like that?
  • Replacing the Li-Ion battery with a regular one is a tricky job; installing a dead switch seems like a good solution for the time being. Does anyone have any other suggestions for how to prevent full battery discharge while parked?


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Upgrade today to remove Google ads and support TLF.

If it's any consolation my V6 Cup also drains it's battery inside 2 weeks and it's a conventional lead-acid clunker. On top of that there's no alarm, no imobilizer, no central locking, nothing you'd think would drain the battery. No idea why it does it or what's draining it but I keep it on trickle charge if I leave it for more than a few days.

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That doesn't sound right... Could it be something wrong with the battery / electric cabling?.. 

Can't help but wonder how my > 5 year old VW can stay parked outside for weeks at a time with the alarm and the immobiliser on and never go flat. As in, how is it possible? 

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  • 4 weeks later...

Hey Konstantin,

can you tell me if it is possible to close the trunk of the car with a power cable connected to the charger running out of it? I have power on the wall of my garage, but I don't like to leave the car open while charging. Also I think there should be a way to open the car if battery runs out and you locked it. Any info on this from others?


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Hey Cristiano. Sure you can, just run the cable a little further up, under the dark plastic (or carbon fibre, depending on your spec) bit of the boot. There's enough space there even for a pretty thick cable.

The electronic central locking system and manual locks are not connected, which is a really bizarre design decision but oh well, Lotus must have had reasons.

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Hey Konstantin,

Thanks for the info. Good to know that I can run a cable from the engine bay, basically I will have to remember to put the cable before closing the car.

Two things looks still strange two me from your story.

1) I read that on the S2 Exige was possible to remove a pannel on the front of the car (passenger side) to recharge a dead battery.. given that Lotus is aware of the issue, I would be surprised if a similar solution is not there on a 350 or 380.. maybe in a different location?

2) You say battery is hard to remove: but it should be kind of doable with 2-3 screw removed, isn't it? In the trunk it is kind of easy to access..

I wish I antake a look in the next few days.. but unfortunately my car is still not here... :(

Cannot wait much longer!

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1 hour ago, Cristiano said:

1) I read that on the S2 Exige was possible to remove a pannel on the front of the car (passenger side) to recharge a dead battery..

That's what I've been shown on my Exige when collecting yesterday, exactly as you describe. :-)

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What's the issue with jump starting a li-on car battery?

Would one of those emergency li-on batteries on Ebay work? I've used one on a totally flat lead battery to start my car.



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@Cristiano S2 Exige is a different car altogether and has more in common with Elise than the V6 generation of Exige. I've spoken to 3 different Lotus mechanics, including one working at Hethel, and the only practical suggestion anyone has given me was to smash the glass. Saying this, if you find a solution then please let us know! I'd really like to keep my windows intact.

@DanR Frankly, I don't know. The manual supplement says that Li Ion battery should be jump started only as a last resort and should otherwise be avoided. A few engineers I've spoken to said it's a big no-no.  My guess would be it's something to do with the risk of either frying the ECU or the battery exploding, but I'd be curious to know the exact reason.

I wish there was some kind of portable trickle charger....

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I had a friend who was an electrical engineer and I was talking to him one day about not having the right jumper leads to jump start an ECU controlled car.

He said that was a myth and that all you had to do was to turn on the lights and the fan and the radio if you wished and this would prevent a surge to the ECU and not harm it. He said the fear thing was a very good way for the companies to sell a new type of jumper lead to everyone.

Whether this all still holds true for a Li-Ion battery, I don't know.

All we know is that when they stop making this, we will be properly, properly sad.Jeremy Clarkson on the Esprit.

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Surely you could put any old 12V lead acid battery in, even using jump leads if required, after disconnecting the Li-ion battery, just to start it and move it ?

I realise it may not fit but it could just sit in the boot


Edited by gregs24
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