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Notes From A Small Battery

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I have replaced the Varta B18 battery originally fitted to my 2013 Elise S Club Racer with a lighter absorbed glass mat (AGM) battery, the Odyssey PC680.  This change saves some 4.5 Kgs, including fittings.  (I understand the PC680 has previously been fitted as standard on Cup versions of some Exige models.)  The weight reduction is not huge, but the saving is about the same as the difference between the standard soft top and a hard top - so the latter can now be fitted without reducing performance! These notes may interest others. They include only what I see as the essential technical details. These are my own results - no guarantee that they are general!


1. Having a suitable battery charger is vital.  The “standard charger” used for the common type of lead acid battery is not suitable. For example, even the most sophisticated models sold by Halfords are explicitly stated on their website to be unsuitable for AGM batteries.


2. Odyssey recommends maintaining the battery at high states of charge; it seems usual to link up a charger whenever the car is unused, unless the battery is then disconnected. The charging requirement differs according to the initial state of the battery. The charger specified by Odyssey for the PC680 is their Ultimizer Charger, OMAX-6A-1B (6 Amp). This provides for three phases of charging. See
Although the PC680 battery itself is readily available, I was not able to find a UK source for the charger. One US supplier’s website stated it could supply that item only within the US.


3. Enquiring about chargers, full technical knowledge was hard to find so I made some measurements of my own. I identified what seemed - and have found to be - a suitable charger: the CTEK MXS 3.8.  One useful contact informed me that he had satisfactorily used that charger with the PC680 over several years. It is able automatically to charge from a range of initial battery states, using up to six different current/voltage charge and test profiles and includes a suitable medium term trickle charge at 13.6 volts.  The PC680 trickle charge voltage must lie between 13.5v and 13.8v.  (My “standard battery” charger provided only 13.2 volts.)  Also, for durations of weeks or months, if a battery is supporting the drain of the components that remain live when the car is locked, the CTEK charger cycles the battery between full charge and a slightly discharged state.
4. To assess the effect of not maintaining a trickle charge, I left my car locked and immobilised for 100 hours. ( No hardship - I was away for four days!)   The charge state declined to 85% of full.  However the engine still started immediately with no apparent difference from a full charge. (Odyssey provides a scale showing the linear relationship between voltage and charge state.)


5. I also explored the situation when the car is being driven. Two aspects are important. First, Odyssey specifies that it is imperative that a charging voltage does not exceed 15.0 volts. Second is the question of  whether the Elise charging system is adequate, rather than being designed only for standard batteries. In my case I wished to consider drives of up to say 10 hours without the possibility of an immediate recharge - ferries, even on a 12 hour passage, do not normally provide mains power!  I had a concern that if the Elise system did not provide at least 13.5 volts there might be ongoing discharge of the battery - unimportant on “short” runs of course. Disappointingly - and surprisingly, as I wanted only the specification of the charging voltage characteristics from the Elise system and avoided asking any “is it suitable” type question - Lotus’s electrical department could provide only the limited information that: “Normally, the (100Amp) 12V negative earth type alternator as fitted to the Elise 2ZR engine has a charge voltage of between 12.5 to 13 volts.” They referred me to the battery maker for anything more - pointless as it was the car system that I was enquiring about.  Lotus’s statement indicated that indeed the battery might discharge on a long drive. (Albeit I do not know of PC680 behaviour at lower voltages.)


6. To clarify matters, I made a number of measurements with a digital voltmeter.  Only a few are relevant here. They apply to the car in operation with the PC680 initially fully charged. (Metered voltages were displayed to 2 decimal places; I have rounded the results to 1 place which should more than cover any inaccuracies in the meter.)

a) With no ancillary equipment on: at idle: 13.8v; at mid to peak revs: 14.0v.  The voltage held within that range through a number of engine speed conditions.
b) With aircon on, the fan at maximum and headlights on main beam: at idle: 13.0v. (Note that aircon gives higher idle revs.)  At up to peak revs: 13.0v. (The same as at idle.)

7. So it seems that the “Lotus stated” charging voltage maximum of 13 volts applies only under peak ancillary load.  Without ancillaries it is close to, but does not exceed 14.0 volts. Thus, without using ancillaries, the PC680 is possibly more likely to be overcharged than undercharged.  Usefully though, the voltage is controllable by use of ancillaries.  Importantly, the maximum charging voltage does not approach the danger level of 15.0 volts.  The maximum reading I obtained was 14.03 volts.


8. I devised my own mounting for the PC680.  The Lotus part costs substantially more than the battery!  Also I wanted a minimum weight approach. I used only the original battery mount holes apart from adding one small one through the engine bulkhead to the washer bottle mounting plate. It is very secure and light and can easily be returned to standard.  I use only the inbuilt terminals on the PC680 and have permanently attached the plug-in connector supplied with the charger. (Details if requested.)

9. In routine use, all is very satisfactory. (Naturally I am conscious of the improved acceleration!) I will use the voltmeter on longer trips for a while - and update these notes if I suffer a flat battery!

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Bibs, an approach for those with a smart phone, I guess, although I don't have one. The comparable app for my iPad is DashCommand I think - must get around to trying that for all sorts of things. Not certain, but it would be surprising if there is not an OBD socket.


Measuring the running charging voltage does not need great precision, but it is useful also to be able to assess the state of charge of the battery and that is shown by quite small (linear) variations in voltage.  E.g. 50% charged shows as 12.20 volts, 100% charged as 12.84 volts.  (Figures at 25 C and after 6 hours without charge or discharge.)


So making meaningful measurements is quite a precise business and I wouldn't want to bring in any more complications or potential error sources.  Think I will stick with my digital voltmeter connected into the interior power socket for charging volts. It could take quite a while before I have the full picture - much irrelevant to most users of course. Driving without a high ancillary drain should keep the charging voltage above the effective trickle charge voltage of 13.5v and I have tested that left parked, locked and immobilised without a charger for four days gives no problem at all.  For longer periods I will leave the charger connected though. 

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  • 4 weeks later...

A further point re the Elise circuit voltage when the car is driven. As it happens I have not yet driven for more than about 20 minutes continuously, but I can add to my experience with the digital voltmeter "installed" via the interior power socket.


With no ancillaries on - which reduce the voltage as reported above - and starting with a fully charged battery apart from the single engine start to get going, the voltage initially is as I gave above.  However after some 10-15 minutes, at "driving revs" the voltage gradually reduces to around 13.7 - 13.8v. I speculate that this relates to the battery charge state increasing but can't say more.  It is possible that the alternator controller senses the charge state as the CTEK charger does, but I would be very surprised at that given the many other current supplying duties it has!


However my concern about overcharging the battery is reduced although I shall keep a careful eye on the volts when I do a longer trip.


Oh for a +/- ammeter as used to be a standard fitment!  All batteries once needed careful management!

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  • 2 weeks later...

Interesting stuff

I'm going to get this battery as I want my CR to be the lightest possible spec (I have no options, deleted the airbag and have TRD airbox and have forged wheels coming).

The battery is about £320 from lotus including the bracket, battery box and fitting kit.

Could you post a photo of your installation please? That would be her helpful


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Ken, will do soon but very busy today/tomorrow.  Note you need appropriate charger too.  I paid £86 for battery (inc delivery) via internet source - will look up - and made my own mounting kit for £5 from “bits and pieces”. Charger was about £70.

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ODYSSEY BATTERY  Notes on my fitting




Refer to Photobucket:



It was tricky to take photos at the end of the boot under the rear clam; the camera was forced very close. Hence there are distorted perspectives and angle effects - but without removing the clam .............!


Properly this would be called a prototype installation; somewhat rough-edged as it had to be designed, produced, tested and fitted in one pass. But it is functionally fine and very secure - no apology for lack of a nice finish. Looks better from a normal distance!
Photos show the details; my build sequence is described last.


Photo 1


General arrangement and showing the following. Some details in later photos.


Battery clamped vertically to rear bulkhead in similar position to standard battery. “Wavy rubber” carpet underlay (not seen) between battery and bulkhead and wheel arch. White clamping board cut from kitchen chopping board 6mm thick; secured each end via fixings to original battery clamp bracket holes by existing bolts. Projection at RH end of board allows charging connector clipped over as shown. Length of 8mm threaded rod approx 130 mm long fed through white board and piece of garden hose up to the engine bulkhead with the rod passing on through a new hole in the bulkhead and the metal mount for screen wash bottle. Secured by nyloc nut each end. (I removed screen wash bottle before drilling!)


Transparent insulating cover cut from large plastic domestic food container;  vertical tongue at RH end of cover runs down to be a firm fit between the battery and the green hose.  LH end of cover slides between the green clip and rear main clamp.  White cable clip bolted to side of cover to guide charging connector cables. Holes for electrical connector cables in the cover, large enough for the cover to be manipulated free and pulled back over the cables to allow access to top of battery.  (How some subsequent photos were obtained - nothing was undone.)  Cables connected to the battery at angles to facilitate removal of cover.


Photo 2


Shows the RH of the white board fixed by an adapted shelf bracket bolted to it and held down via the threaded hole for standard battery bracket; slot in bracket allows white board to hold the battery firmly against the bulkhead. (Standard bracket removed - saves 90 gms - Lotus removed the rear badge to save 60 gms!)


Photo 3


Shows LH end of the white board clamped against the battery using the original battery clamp in its original bolt hole but rotated to the vertical.  The green square piece of aluminium has a lip at one edge and an elongated hole to allow it to be bolted over the clamp and positioned to give a narrow slit between the lip and clamp to hold the transparent cover when in place.


Photo 4


Shows the nyloc nut on the threaded rod protruding through the washer bottle mount. The bottle fits normally.


Photo 5


Shows the top of the battery with the cover removed and the battery cables and permanent connectors for the charger bolted in place.


Practical construction


My assembly sequence: rotated the existing LH clamp (0.5mm filed from the lower end); positioned the battery and underlay pads; clamped LH of the roughly sized white board in position against the battery then marked and cut it to final size; mounted the RH bracket to the white board and bolted it into the original bolt hole; selected appropriate hose (thick walled for some flexibility); drilled the white board for the threaded rod to fit the hose snug against the battery and slightly compressing the underlay at other end (could have packed if too lose or elongated hole if too tight); Inserted threaded rod through white board as guide for new bulkhead hole and drilled; roughly sized and cut the transparent top cover and positioned it by tongue on RH side and clamp slot on LH side; marked and cut holes in the cover for electrical connectors; assembled all and fastened electrical connectors semi-tight so angles can be adjusted to allow the cover to be moved off and on before the final tightening.

Edited by mdavies
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  • 6 months later...

Glad Lotus have now seen the light for the Exige Club Racer announced today.


For those that think a terrible pun is no excuse for a post, if the same brand of light weight battery is used as the one I address above - an Odyssey AGM -  my experience may be useful.  Probably an Exige battery will be of a larger capacity though and so its likely a higher current charger would be needed - or at least desirable. The voltage aspects will probably still apply.


I trust Lotus's electrical department have now put their act together regarding it! (See above.)

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