Lead acid batteries
The majority of classic motorcycle batteries are of the conventional lead acid variety. Whilst we think of these as being either 6 or 12v, this is a slight misnomer as the actual voltages are rarely these values if the battery is in good health.
The primary battery voltage is dictated by the chemistry of the lead acid cell. Fully charged and in peak condition, each cell has a voltage across it’s terminals of 2.1v. Motorbike batteries are made up from a combination of such cells connected in series to give the required voltage.
A ‘6v’ battery is comprised of three cells giving a total voltage across it’s terminals of 3 x 2.1v = 6.3 volts. Likewise a ’12v’ battery is made up of six individual cells giving a total of 6 x 2.1v = 12.6 volts.
These voltages are not however fixed and will vary both with the state of charge (or discharge) of the battery, and also depending upon the electrical load placed upon it. The more the battery is used and loses its charge, the lower it’s voltage gradually becomes. Similarly, when an electrical load is connected to the battery this also drags the voltage down.
State of charge
It is therefore possible to determine the approximate state of charge of a motorcycles battery by measuring the voltage across it’s terminals. This is how ‘intelligent’ battery chargers work by constantly measuring the voltage and stopping charging once the desired level has been reached.
The table below shows the approximate voltages that will be measured across the terminals of a lead acid cell for various states of charge. Also shown are the total voltages for 3 cell (6v) and 6 cell (12v) batteries as will be fitted to most classic motorcycles.
Note that the voltages shown are open circuit voltages which means that there is no electrical load placed upon the battery. These readings are therefore taken with all lights etc turned off, and the engine not running.
|State of charge
The voltage applied to a battery is also very important when it comes to maintaining the condition and chemistry of the lead acid cells and ensuring peak capacity. The table below shows the correct charging voltage ranges for individual cells as well as 6 and 12 volt batteries for various different modes of charging.
|State of charging
|Minimum for charging||2.15v||6.45v||12.90v|
|Trickle charging||2.25 – 2.27v||6.75 – 6.81v||13.50 – 13.62v|
|Normal charging||2.30 – 2.35v||6.90 – 7.05v||13.80 – 14.10v|
(risk of gassing)
|2.40 – 2.45v||7.20 – 7.35v||14.40 – 14.70v|
Fast charging is not recommended for daily on-bike charging as it results in gassing and requires careful monitoring and possibly topping-up of the battery. Some intelligent battery chargers may employ such high charge rates for part of their charging program as this can help to maintain correct battery chemistry. However it is not recommended for normal charging from the bikes on-board electrical system.
The normal charging voltage range is how the majority of battery chargers and a classic bikes on-board charging system will feed charge into the battery. The minimum charging voltage for a lead acid cell is 2.15v; a little more than the cells own voltage of 2.10v.
Lower charging voltages are useful for trickle (maintenance) charging whereby battery condition may be kept topped up whilst constantly connected to an appropriate mains charger when not used for long periods. In this way the battery will always be fully charged and ready for use.
Selecting a replacement battery
There are a number of different types of battery available for use on a classic motorcycle ranging from the traditional ‘wet’ lead-acid unit, to sealed units and on to modern maintenance-free ‘valve-regulated lead-acid’ (VRLA) batteries such as ‘absorbed glass mat’ (AGM) and gel types. There are also the ‘Cyclon’ cells which are also proving very popular.
With so many different types available it can be difficult to select which one is the most appropriate for your classic motorbike, but hopefully my ‘Selecting a replacement battery’ article can offer some useful guidance.[sc:disclaimer]
My 8Ah battery is charging with 16V at 1A. From what I have read the amperage is OK for a normal charge (4A is quick charge) but the 16V is way higher than experts suggest. The battery is gassing – should I continue charging or am I doing damage?
Hi there. The 16v you describe is definitely too high for charging a 12v battery – 14.4 volts is a suggested maximum. Gassing is bad for the battery, and also potentially for you as the gasses are explosive! You don’t say whether this is on the bike or on mains charge? If on the bike, the regulator needs adjusting to reduce the charge voltage. If a mains charger, well think this might need replacing!
I’d stop this charging, leave the battery to settle in a well ventilated space and then top up the fluid (if possible) to the correct level with deminarelised water. Check the voltage against those in the table above to get some idea of it’s state of charge, then charge at the correct voltage if required.
Hope this helps! Regards, James
Thanks James, it’s a sealed battery – does it make sense to unseal it now and check the levels? The standing charge is 12.6V at the moment so it looks OK.
If it’s a sealed battery then no need to top up. I just assumed it wasn’t sealed when you mentioned it was gassing. Voltage sounds good so hopefully no damage done, but best to avoid such high charging voltages in the future or battery life expectancy could be much reduced.
That was a great breakdown of charging etc, just what I needed. I’ve suspected my battery to be dying as it’s not much over 12v each morning and the lights dim when I brake etc. Charging voltages are ok so a new one may be in order.
Hi James, Nice job on this page. Since you understand 6V batteries, hope you can help with my RV bank of 4 Xs 6V batteries wired to output 12 Volts. Each battery has a 2 AR rate of 146 and a Mfg recommended Float Charge Rate of 2.20 V. Soooo Question is if I hook-up a 1 Ft squarish solar panel to this bank ( which puts out ~ 18V @ 175mA @ 3W ) and leave it alone for months at a time in Arizona ( where one can really cook an egg on the side walk, for 9 months out of each year ) will I over charge or not do much at all?!? Thanks in Advance Paul
Hi Paul. I’m not particularly familiar with RV battery and solar panel setups, but I would suggest that you need a charge controller between the PV panel and batteries in order to regulate the charge. If the recommended float charge is 2.2v per cell, that’s 3 x 2.2 = 6.6v for each battery and 2 x 6.6 = 13.2v for the series setup. Applying 18v is not going to do the batteries any good, so you’ll need a charge controller to regulate this to the correct charge current, and also to disconnect the batteries when the suns not shining to prevent discharging. Hope this helps, regards, James
Thanks James for an informative article. I’ve knackered two new battery’s in very quick succession. They went flat very quickly and now they won’t hold a charge. The only problem I can find is the battery is getting 9v when I rev the engine while the bike is running. Is this too high and will it kill batteries? The battery fluid doesn’t seem to fluctuate.
Hi Craig. You don’t say whether your bike is 6 or 12 volt. If it’s 6v then 9v will quickly fry your battery as the max charging voltage should be just over 7v. If your bike is 12v then 9v is no where near enough to charge it. Check out the above ‘charging voltage’ table for more info. Either way, it looks like you have found your charging problem (the regulator)!
Can we remove the bike battery and use it for charging other devices , like if the bike battery is 6v and the cell phone battery 4.5v
Hi. Got a mystery on 12v Triumph twin single phase alternator. Shows no charge at amp meter but noticed lights got brighter with revs,
so put voltmeter across battery (pos Earth) spot on up to 14.55v Tried another ammeter no joy both show discharging when lights on but no charge.checked wiring in series neg to ignition switch all seem OK. Any ideas please. Gratefully received.
I bought a new chunky 12v sealed lead acid battery made by Westco a couple of months ago for a Harley 1340cc bike.
After using the bike only a couple of times in January I took the battery out and charged it then disconnected it from the charger. A week later it measured just 12.4 volts. A week after that it has risen to 12.5v (the battery being off the bike still).
What would you think of these readings given the circumstances?
Hi Andrew. I wouldn’t take much notice of the difference between 12.4 and 12.5 volts. That’s probably down to multimeter accuracy, the way the meter was connected, different battery temperature, or other such second order effects. However I would have expected a new battery to hold 100% charge for a bit longer before dropping to 75%.
Is there a date on the battery? Sometimes you can buy old stock which has sat on the shelf for months or even years and hence the battery chemistry is already old. Try charging and fully discharging the battery a few times (leave sidelights on overnight to discharge at a slow rate, headlamp would be too quick), and that should help restore and maintain capacity by refreshing the battery chemistry.
Regards, James 🙂
I have a problem with th wiring on a
Go 550 a new ignition as been fitted when I connect
Them 12 volt battery in some areas there headlight I’m only getting 6 volts
Any ideas what it might be thank you in advance dave
Hi Dave, Not really sure what bike you have there, but if you are only measuring 6v anywhere on a 12v electrical system then there is something very wrong! Even a flat 12v battery would give more than 10v. Check the voltage at the battery and then work along the wiring to see where the volts seem to disappear.
I think you will find my new book useful as it has a big chapter on diagnosing just this sort of problem: http://www.amazon.co.uk/dp/1847979955
Kind regards, James 🙂
I’ve got an 07 wr250f dirt bike which has had a few issues. At the start of the day the bike sits at about 12.8v dc. I rode it all day yesterday and it was going fine (I’ve had a few battery issues so I just hook up my multimetre whenever we stop to suss out how it’s going) it I did notice during the day it dropped to 12.6v, and then to 12.25v just as we called it a day and went home. Any suggestions why it may be good down in charge during the course of the riding day?
Really helpful no bullshit posting,very good thanks a lot!
Hi james, I know this thread was started a while ago, but have you got any idea why my motorcycle battery would be reading around 18v whilst idling? I bought new reg/rec and wired it up today to see if it made a difference, and when I ran the bike, the voltage was reading high 12’s whilst idling with choke, before gradually climbing up to 18 again. Doesnt sound like reg/rec… any other ideas?
Would hugely appreciate your views!
Hi Harry. You don’t say what bike and electrical system you have (dynamo or alternator?), but it does sound like a regulator issue. It pretty much has to be a regulator issue by definition I think if the voltage from the generator is not being properly regulated! What type of reg does your bike have? If it is an original electro-mechanical type, then it could well be that your replacement is as faulty as the original one. Even the modern ones tend to need some adjustment to get teh voltage output right. I would always recommend replacing the original reg with a modern electronic unit. It will give you better charging efficiency and much better control of voltage output. Don’t run the bike with 18v going in to the battery as that is sure to ‘boil’ or over-charge it. Worth checking the electrolyte levels if the battery isn’t a sealed maintenance free type.
Hope this helps, James 🙂
Hi guys, I found this post in a search trying to figure out my own issues, hope someone here can help.
I have an 87 HD sportster, the battery is about 3 years old, was on a battery tender plus most of the winter. Bike started up with no issues yesterday. Ran a few minutes and stalled. Started up again, same thing, a few minutes, then stalled. Went on for about 4 or 5 times. Then the battery started ‘clicking’ (dead battery sound) after about the 5th times starting. (this bike has starter stall so I always thought my battery was low until I understood the 87 technology)
I put the battery back on the tender for about 30 minutes then started it up again, no problem and disconnected the negative cable.
Bike started to spit a bit and studder and eventually stalled within about 15-20 seconds.
Took the battery out and tested the volts, got 13.11 volts.
One person said it’s a bad battery, probably an open cell, 1 cell is bad.
My question is the voltage regulator bad as well? By disconnecting the negative cable, if the regulator was bad, would the bike stop right away? or does the regulator hold enough energy to keep it going for 15-20 seconds? This set up is with a regulator and a stator.
Any info would be greatly appreciated.
Bought a deauville 700 and got a pair of small led running lights and connected them to the two side light wires.promblems started there now running them off the battery with fuse and switch,but battery went flat tried charging it a few times but every time I start the bike get a series of clicking on the main 30amp wires.Sealed battery reads 12.7 after charge and did just turn over before series of clicking,taking the battery for a test and showing 12.7v he said no amps it’s cooked I’ve got a fluke meter tried testing for amps on the battery without any joy.
This is idiotic. I just got a new 6v lead acid battery for my ’47 Indian Chief. My battery charger doesn’t charge low enough so I thought I would use a battery tender. I have 6 of them for old and newer bikes. I looked for the one with the lowest charging anps. One charges at 1.5 so I used that one. After about 8 hours I checked it. It was a 12v tender and not 6v. They all look the same and like an idiot I was in a hurry. anyway it reads 7.1v right now. The battery was warm but not hot. The tender was hot! Question; did I ruin the battery? Will it leave me stranded someplace?
Hi Barry. Easy mistake to make hey! But you may not have done too much damage…perhaps. What type of battery is it? If it is an old style non-sealed unit, then just check the fluid levels and top up. Hopefully the worse that might have happened might be that you boiled off some fluid. However, if it is a modern sealed battery then there is obviously no way to check or top up the levels. If it some type of more fancy battery (AGM, Gel, or something ‘special’ like that), then you might have done more damage as these batteries are more sensitive to having the correct charging rate and voltage. But I’m guessing it’s just a standard lead-acid you have, so fingers crossed it will be ok. Good luck, and please let us know what happens 🙂
Thanks for the response! It is a lead acid 6V battery. It seems to be holding a charge and I put a 6V tender on it for a while and it seems fine so far. I just didn’t want to get stranded someplace with it. I plan to put it in the bike today and get it running. I will check back with hws it did. Thanks again!
Hi there, quick question. I’ve bought a brand new battery early this year, had it run down a few times and had to get the bike jumped (my stator was fucked) but it has been perfectly fine since. I recently re-painted my tank, and did most of the sanding with the tank still on the bike (so I could sit in the seat, I’m lazy that way) and a lot of dust and water has gotten under the seat and also on the battery. Right after I put everything back together I started getting an issue with the bike cutting out when I was in first gear (only while the bike was cold tho) and one morning it wouldn’t start at all anymore. I assumed it was simply a loose connection on the battery and took it off to charge it, but the reading is still at 12.37V. Is that low enough to cause it not to turn over or do you think the problem is with something else?
Hi James, I have a Kawasaki Voyager 1700. Installing a new audio system with JL Audio amp, replacing factory head unit and an Alpine amp. Shop can’t get new JL Audio amp to stay on. All works fine at idle, but amp goes into protect when throttled up. At idle, battery is showing 14.2/3 and 14.8/9 when throttled. Specs for the Reg/rec show 14.3-14.8 is normal voltages, also there are also 2 ref/rec on this bike!
Both old Alpine amp and new amp are rated at 14.4 input voltage. It’s not a heat issue since new amp shuts down only when voltage increases. Battery is 12v AGM, only 1 month old. Bike has a Stator which I’m told is fine. Any ideas?
Hi Jeff. Thanks for your message but, sorry, I don’t think I can be much help as I’m not familiar with such electrical issues on modern bikes. The voltages you mention seem normal and 14.8/9 volts should make no difference to a device rated at 14.4 volts. Any automotive equipment should be designed to work with a wide input voltage range anyway.
Try turning on the headlight. This should limit the max battery voltage – does the problem still occur? Are you sure the new amp isn’t faulty? Have you tried another one of the same type as a check? From what you have said, (and assuming that there are no big voltage spikes coming through that you have’t / can’t pick up on your meter), then ,y gut feeling would be a faulty amp.
Good luck, hope this helps, James
I have an “08 Suzuki SV650. I connected an extra LED light which draws .1 volts on my Digital volt meter reading that I also installed on the bike. I left the LED light on over 14 hrs which drained the battery down to 9.5 volts. I put my trickle charger on for 24 hrs and also rode my bike for 2 hours which registered 14.5 volts while riding and brought the battery back up to 12.5-12.6 volts when sitting idle. It measured the same the next day at 12.5 volts. When I turned on the ignition, the voltage drops to 12 – 12.1 volts. The next day, I turned on my electric grips which upon turning on my ignition, drop the voltage to 11.8. My bike would not crank over. Once turning off the electric grips, it barely started at 12 volts. Is my battery good, or did draining it down to 9.5 effected the battery’s state of charge. I’m worried it might strand me on 5 day camping trip.
Thanks James, your advice is very sound.
Hi Steve. Sounds like your battery is getting old, but not sure it would be the new LED that is the problem. You say the LED draws 0.1 volt. Do you mean that it draws 0.1 Amps current, or that it lowers the battery voltage by 0.1v when it is turned on? You really need to know how much current it is drawing from the battery to work out how long the battery should last with it on. Do you have a separate multi-meter with ammeter function that you can connect in series with the LED?
Not sure where you are in the world, but it seems much more likely that the cold temperatures have finished off a battery that was on its last legs. A single full discharge won’t have done it much good, but it also should not have done any real damage that a full charge could not repair.
Hope this helps, James 🙂
thank James for the quick response. The Battery is only 6 months old, but I’m not sure how long it sat on the Dealer’s shelf as when I purchased it, he only had 6 batteries of which one fit my bike.
The LED lowers the voltage .1 on my digital meter when it’s turned on. I have an on/off switch for the light. I left the bike alone for two days and it still reads 12.5 volts and started up just fine. Once I turned on the ignition, without starting it, the voltage dropped .5 volts. I rode my bike and it stopped and started just fine. I do have a warranty on the battery but afraid it will test out just fine. Should I replace it none the less?
Hi Steve. Hard to say if the battery is good or not, but if the weather is cold and you are using accessories like heated grips, then you’ll want it to be very good. (Having said that, best to only turn on heated grips when the engine is running as they otherwise draw a lot of power from the battery). Personally, if I had any doubt before going on a big trip, I’d probably replace it, just to be sure. Good luck, and have a great camping trip! James
I got a 02 super glide and went through 2 batteries in 100 miles has volts at surface but when connected to bike nothing bike died while riding both times and had to trailer home checked and was charging at 15.01 at a hard rev
My 12v battery is showing around 8V which shows it is discharged. On starting my bike, the battery shoots up to 12.34 V. I know battery is discharged, and it is not taking charge. However, I am not sure if battery is dead or problem is with the battery charging system. Is there any other test to confirm this.
My 12 volt 35/35 watt head lamp system started decreasing battery voltage and incurred hard re-start condition once I modified my headlights to receive 55 watt H1’s….possibly regulator on this 250 cc cannot produce enough voltage…I just ordered some low watt LED’s and have a new battery i’m going to charge and retry before installing LED’s…your thoughts are..? (thanks)
hi Steve, i have some questions, please reply if possible..
1. Want to change battery with lithium ion batter 3.6×4=14.4v 2-3ampere for replace my 12V 2.5 amp battery, is it make any problem to my accessories ran from the battery [like horn, head light, indicator light, meters/dashboard several lights???
2. i want to know the current produced from this bike??
bike is Yamaha 106cc libero.
Great no nonsense pages, just what I wanted, thanks a lot and well done !
We have a V-Star 950 let it sit too long and battery seems to be dead I only have the mini battery tender should go ahead and get a regular 12v charger to try abd re-charge the battery
Thanks for the article. I recently bought a 1964 Triumph TR6. It’s a 6v system and the battery is usually around 6.10 to 6.20 when I test it with a multimeter. The bike starts on the first kick and runs great so I’m assuming the charging system is working okay. If I hook up a multimeter to the battery while the bike is running, what kind of readings should I be getting… while idling and then while revving the bike? Thanks.