Which Battery ?

[Tech01]

Several threads recently have asked about vehicle batteries – what makes a good one, how to tell different ratings apart, diesel vs gasoline, batteries going flat when the vehicle is not used regularly, etc.

What follows is all pretty basic stuff, and I apologise to those who know it all already!

Battery rating

Vehicle batteries are rated for their voltage (usually 12v), capacity (e.g. 70Ah), and cold-start or cranking current (e.g. 680A). What do all these mean?

Voltage is the straightforward bit. Your vehicle’s electrical system is designed to work around a particular voltage, usually 12v. Some ancient vehicles used 6v, most trucks use 24v, and a few very new highly-electronic and hybrid cars use a new standard of 42v.

So, you’d like a Battery with plenty of Ooomph, that will start the vehicle easily (particularly in the cold), and hold its charge for long periods of non-use. There are actually two quite distinct measures of what most folk call “battery capacity”.

The capacity of a Battery is a simple measure of how long it is capable of delivering a given current. This is usually measured in Amp-hours (Ah). A 70Ah battery would (in theory) be able to deliver 70 amps for 1 hour, or 35A for 2h, or 7A for 10h, or 1A for 70h, etc.

The smaller the current drain (in Amps) the longer the battery will deliver it (in hours). Commonsense, really. In practice, for a tiny current (eg 0.25A, or 250mA (milliamp)), the battery will go on delivering it for much much longer – perhaps double the time. And the converse is also true: at high loads (e.g. 20A, such as four headlights left on), the maths doesn’t quite deliver, because, towards the end, the battery gets ‘tired’, its voltage drops, and the lights go dim.

But the moral is, a big capacity battery will keep things going for longer . . . and, importantly, that includes all the things you can’t switch off when you walk away from and lock up your car – immobiliser and alarm, radio memory, ECU memory, speedo/odometer readings, remote-entry and central-locking circuitry, and so on. The newer the car, the more these add up. So, how much electrical load do all these goodies add up to? Hopefully, not more than 100-250mA (i.e. one-tenth to one-quarter of an Amp). At 400mA (four-tenths of an Amp), you might well have a problem. On petrol-engined cars, which tend to have smaller capacity batteries, a 400mA drain has been known to drain a newish battery enough so that it won’t start after 3 or 4 days of being parked up. This is the kind of drain you’d get from leaving one 5W interior light on by accident.

Right, then. Big capacity (Ah) battery is good.

The second, and very important, measure of how well a battery will perform is its cold-start (or cranking) current. The starter motor consumes huge amounts of current, maybe 200-300 Amps on a petrol engine, and 400A or more for a diesel. Although (hopefully) the starter motor is only pulling all this current for very short periods, the battery does have to be up to it in the first place, i.e. it must be capable of actually delivering such a current. Tiny batteries won’t start diesels, as many of us know.

Things are made much worse when vehicle batteries are cold. The chemical reaction inside is just sluggish at low temperatures. So, there is an international agreement that the starting (cranking) current which a battery is specified to deliver is that which it can manage in the cold, and this is usually at 0degF (-18degC).

So, the battery you’re looking at is marked 12v 70Ah 300A (EN). Is this good or bad?

12v is fine, we’ll assume.

70Ah is the capacity, and this figure is pretty middling-to-good. 50Ah would be quite tiny for a full-sized car, even a petrol one. 80Ah is better, and 90 and 100Ah getting pretty hefty, as does the physical size as well, as a rule. Something around 75Ah is usually quite enough, even for diesels.

300A (EN) ? Now this really does matter. (The EN marking, by the way, is just the Euro standard method of rating and testing these things.) 300A is a pretty low figure for a cold-cranking rating. OK for small-medium size petrol engines, and maybe for very small diesels on warm days, although I wouldn’t recommend it. For a bit of comfort for your petrol engine, go for a cold-cranking figure of 400A or better. For a diesel, go for 600A or better. You’ll find batteries that offer figures up to 800A, but make sure this is a cold-start or cold-cranking figure, and not something else. My own RAV4.2 D4D has a battery marked as 12v 74Ah 680A (EN). No problem, although there is better out there, not much more expensive, but often too bulky to fit. (Always measure up first if going for something bigger, AND check that the + and – terminals are in the right corners!)

If you have the space, always fit a diesel-spec battery, even on a petrol car. Not only will this give you extra margin, but you never know who you might have to jump-start.

So, that’s it, on a simple level. Hope it might be helpful.

Chris

PS So what about those huge Leisure Batteries, for caravanning and boating? Looking along the shelf you might see capacities of 100-130Ah . . . Must be good! Well, no – they’ll keep your caravan or navigation lights going for days on end (a gentle loading for a long time), but are pretty useless at delivering large currents (e.g. vehicle starting) reliably – their insides can buckle under the strain! (Will happily operate a caravan MotorMover, though, but that’s another story.)

[shcm]

and hybrid cars use a new standard of 42v.

The so called "new" 42V standard was touted well over 10 years ago. At the time, cynical ****s like me, who have probably been in the industry too long, doubted it would really take off. For a start, a vehcle manufacturer is going to have to re-design all the bits in his "parts bin". That's expensive and it would probably need one of the big boys to take it on, in order to affect a change. It felt more like a "gravy train" exercise for consultants at the time and I think cynical ****s like me have been proven right, because, I'm still design ECUs for "12V" (actually they are usually designed to work/survive from anything like 5V to 26V DC, plus all the nasty supply transients).

You might produce a higher voltage supply internally for hybrid electric motor drive ECUs.

The newer the car, the more these add up. So, how much electrical load do all these goodies add up to? Hopefully, not more than 100-250mA (i.e. one-tenth to one-quarter of an Amp). At 400mA (four-tenths of an Amp), you might

Sorry, but I really have to challenge those figures. 100 to 250mA sounds excessive to me. Maybe a tenth of that. For ECUs that have a permanent Battery supply (The industry in the EU, very often refers to permanent supply as KL30 KL=klemme - german for connection), vehicle manufacturers are specifying lower and lower quiescent currents. 50u(micro)A is not uncommon for an inactive ECU. Yes, a body controller may be a bit more in "sleep" or "unlocked" states.

I recently rushed together a prototype ECU design. It's quiescent was about 1mA, because I effectively "threw it together" and expected it to not be optimal. There was a bit of grunting from the customer about quiescent being a bit high, but it's OK for now as a prototype ECU on a prototype vehicle. While there are an increasing number of ECUs on modern vehicles, most are not active with the vehicle "parked". With my "thrown together" not production optimised design, you'd need 100 "badly designed" ECUs to reach your 100mA figure.

[Tech01]

Hi shcm,

I agree with you completely. The 42v proposal was effectively shelved, although it superficially came from a worthy desire to lower the current flow in vehicle ancilliaries. 12v is not the best choice for high-wattage devices, which is why trucks don't use it, I presume.

Yes, the quiescent currents on ECUs, memories, etc etc. should be, and usually are, extremely low - at the figures you quote. But, throw it all together - along with possibly poor quality or defective alternator quiescent back-current - and you can begin to approach an unhelpful figure. Personally, I'd be happier to see around 50mA max when parked up, but I've found several miscellaneous vehicles where the figure was much higher, and which failed to start after several days of parkup. To be fair, the first suspect is usually the alternator.

The purpose of writing my article was simply to explain CCA, 'capacity', etc etc. From numerous other posts, it's obvious that many careful owners don't understand what these markings mean, and can end up putting up with less than the best.

Thanks for putting me right on the other things.

Best regards,

Chris

[greenwidow]

I fitted a ‘leisure battery’ on my 2005 vvt as a temporary fix when my original tiddler began to show signs of age. I don’t do the tech stuff but I was astonished at how this Battery performed; suddenly the engine spins over eagerly when I turn the key on frosty mornings. It fits the tray just as a diesel Battery does. It has a 5 year guarantee and is a 100AH deep cycle Battery with engine start capability and is suitable for ‘Cars, Caravans, Motorhomes, Narrow boats, Electric vehicles, Commercial vehicles and Renewable Energy systems’. They don’t buckle under the strain of starting engines as they are designed for that too. You can find them on eBay for around £100 delivered. I fitted the Elecsol 100. You can read the tech stuff here: http://www.elecsolbatteries.com/products/Leisure-Batteries

[Tech01]

Thanks for this. The Elecsol batteries have been around for some time, and have earned a good reputation. There is a significant market for batteries that will double as vehicle-start and leisure, for the likes of motorhomes and boats. Traditionally, it's been necessary to choose one or the other, but the Elecsol range aims to do both.

I looked through their website, but couldn't find a figure for CCA (cold cranking amperage), which I would have expected to be quoted. Perhaps you could unearth this.

Chris

[sniper9652]

-

[sniper9652]

Not long after getting my 54 4.2 XT4 D4D I was having trouble starting, I identified that she had the original Battery on and after 7+ years of use it was time for a new one! I did loads of research on type, size, Ah, CCA (cranking power), price....etc! I finally ended up choosing the VARTA G8....it fits perfectly in the tray with the terminals in the correct position! The difference is unbelievable, she cranks over and starts with the first turn of the key even after no use over a cold weekend! It may be my immagination but you can feel the power it produces, and it sort of brought the car to life!!

Type: VARTA BLUE G8 12v

Ah: 95

CCA: 830

After shopping around on the net I got one for £90 and collected it from a Battery shop in Stockport! 2 months in use now and it is prooving to be a great buy!!

Steve

PM me for a link to the store and the Battery info if you wish! (It is up for £106 but if you collect he will take £90...fair one me thinks!!)

[greenwidow]

I can't find the CCA on the label of the Battery, it may be hidden on the back but I guess you can contact them for this info: technical@elecsolbatteries.com

It would be a bit of a fiddle to get the Battery out right now as you'll appreciate. I can tell you that the Battery performed perfectly last winter at -15 and the engine always started immediately so I can't say how long it would crank a difficult engine.

PS. I don't work for them and had never even heard of them until I got hold of this battery, but I will use them again.