This page is about BMW motorcycle batteries, but
applies to most lead acid batteries.
Which cable should be removed first?
A lead acid battery can produce hydrogen gas. It is extremely explosive. One
must use procedures to insure safe operation around a battery. Here are a
All current drawing devices (ignition, lights etc) should always be turned
off before trying to disconnect a battery. One wants no current flow to assure
that there is no spark as a cable is lifted off. A circuit malfunction may exist
and be drawing current all of the time. There is no easy way to know that
without having the cable disconnected.
If one removes the positive (ungrounded) cable first and accidentally allows
the tool to touch a ground, sparks will fly. If some circuit is drawing current,
then a spark can occur. For that reason it is best to always first remove the
cable that is grounded and that is usually the negative one. If one wants to go
further, remove it at the ground end, not the battery end. Any spark will then
be away from the potentially explosive gasses of the battery.
Remove the negative first and attach it last.
Connecting the charger is also dangerous. I suggest connecting it while the
charger is unplugged and/or turned off. I like to flash the two clamps together
first to see that they don't spark. I have been surprised more than once in 45
years of doing this. Don't worry, at least with the older analog type charger,
you won't damage it by a fast flash. On the new "smart chargers" one would
damage the electronics. Check the clamps once they are on the battery
terminals by wiggling them a bit before turning the charger on. If your charger
has a lower setting, always use it first to check for a low current flow. I like
to start with the Variac set at zero.
Frequent battery inspection
This one is simple. Inspect the terminal connections for corrosion and frayed
wires. Make sure that it is fastened down securely. Take a look at the level of
the acid. Do not allow the acid level to be above the line or the plate. If the
acid level is too high the battery has a greater chance to boil over and onto
the frame. I highly recommend using dielectric
grease on the terminals. Any type of grease is better than dry.
Vaseline would work well too.
Battery removal, /2 (late 55 thru 69)
Remove the air cleaner, wide rubber battery strap and the wires. The battery
strap can be a pain to remove. Grab it from above and one hand on each
side of the strap and push down. The battery will come out forwards and to
the left easily. The battery in the early Earles fork bikes was 90 X 80 X 162
mm. In mid 57, for the USA only, it was upgraded to 120 X 90 X 165 mm
high. At that same time, many other changes
were made for that series.
Battery removal, /5, /6 and /7 (1970-80)
Generally the battery comes out the top. Don't waste the time to remove the
air cleaner and associated parts. The two bolts holding the rear frame section
onto the main frame section may need to be loosened or removed for the larger
battery on the LWB models.
Trickle charge your BMW battery
At my age I know a lot about trickles, and something about BMW motorcycle
battery trickle chargers too.
The only difference between a trickle charger and a BIG ASS CHARGER is it's
charge voltage while under load. With a higher charge voltage, the battery will
draw a higher current. It is the charger's ability to deliver that larger
current that makes it a BIG ASS CHARGER. A charger that has a high
voltage, but no capacity to deliver the current, quickly has it's voltage drawn
down to a smaller amount and little current.
It is easy and safe to buy a trickle charger designed for a motorcycle
battery. They seem to cost as much as a car charger. Why is that? Why waste the
money? I bet that you already have a charger. One of my tricks is to use any
charger, of any capacity, to trickle charge. With all of the new exciting
technology we can now go to a flea market and buy the once expensive "old
stuff", for nothing. I use a variable transformer, called a Variac, to adjust
the mains voltage that the charger plugs into. Normally you are stuck with
whatever is available at the wall outlet. A Variac will use the "wall voltage"
and adjust it to any amount from 130 down to nothing. If you have a charger with
an ammeter that you trust, you can use it to trickle charge. Here is how I
This is a very small Variac, but plenty large enough for using to adjust the
current on a typical 6 amp charger. This one came out of some type of electronic
equipment and needs to be wired up to work. I usually get these for $1.
The side view showing the connections for input and output.
This is the back of the Variac. The legend shows which lugs are for what and
the capacity of 1.75 amps. That is at 117 VAC or about 200 total watts. See the
three lugs on the right side? They connect to the ends of the circular wound
transformer. At the left side is the wiper (adjustable part) that makes
connection to the various windings. Make sure that you find no broken wires on
the transformer, as that is about all that can go wrong with this type of
transformer. This type of Variac needs to be mounted on something to protect it
from shorting out on something. I use whatever box I have laying around. I found
these are really useful if the charger has room to mount this small Variac
inside of it.
This is a slightly larger Variac that I mounted in a sort of box with a
switch, AC meter and an outlet for the charger to use. I use this one for
general electronic work that may require anything from 0 to 130 VAC. This
means that I can take the wall voltage, about 120 volts AC, and increase it
slightly or decrease it to zero.
This is probably the best deal for a trickle charger. It is commercially
made. It has the cord, a somewhat calibrated dial, on/off switch, fuse, outlet
for the variable voltage and an "on light". This one is 10 amps, much larger
than needed, but is very convenient to use. It is ready to go. It is
also more expensive and heavy.
Plug the Variac into the wall. Plug the charger into the Variac. The charger
should have an ammeter built in or one should be added to one of the wires going
to the battery. Attach the charger wires to the battery. Adjust the current
reading on the charger by turning the big knob on the Variac. That's it.
It is really easy.
The enemy of charging is heat. Go back in an hour and put your hand against
the side of the battery. You should not feel more than a few degrees of
temperature increase. A trickle charge will take 24 hours or so. Again check the
battery temperature. It may feel slightly warmer than before, but only very
slightly. If the battery is hot, then you know that it was being overcharged.
A slower charge is safer and will help the battery life too.
A normal charge rate is 10%. For a typical BMW battery of 20 amp hour rating,
that would be a charge of 2 amps. After 12-24 hours you should see that the
current has tapered off to less than one amp. The battery is charged up to its
capacity. To trickle charge it, use even less current. I would suggest about 1/2
amp for at least 24 hours. The current won't taper off so much with a trickle
charge. The reason is that even with a full charge, a battery will continue to
take a small amount of current. The idea is to start with a current somewhat
above that minimum current and let it taper down slowly.
An old 20 amp battery no longer is a 20 amp battery. The capacity start
decreasing on the first charge. It tapers off slowly to the point that it no
longer will crank the starter. It may now be a 5 amp battery. To charge it at
10% would mean 1/2 amp. Your old used battery is of an unknown capacity and it
is better to charge it at less than the original amount.
I actually use this Variac method more often to charge bike batteries at the
normal rate. A charger for a car can easily over charge a smaller battery. With
a Variac, I can adjust it to whatever I want. Variacs are available in much
larger sizes too, some weigh 100 lbs. Some are for 240 VAC, so check it before
buying. For three phase power they stack three on the same shaft, one for each
An example of filling and charging a new BMW motorcycle battery.
This battery is for a /2 that is getting converted to 12 volts with the MZ-B
kit. It just barely fits into the space for a battery on the /2 twin.
This battery came with the acid in a container specially designed to allow easy
filling of the battery.
This shows the battery and acid container.
The black plastic cap is removed and kept. It does not seal the acid in the
container. The acid is sealed in with a piece of metal tape. Then I set the acid
container on top of the battery and push it down. The battery has a knife in
each cell that automatically punches a hole in the acid container. Now the acid
starts draining into the cells. It will take several minutes for it to all drain
into the battery case. This is really clean and simple when compared to the old
way of doing it. The chance of acid spills and damage are greatly reduced.
I have done this twice just recently and not one drop of acid was dripped onto
The acid has drained in and the battery is now full. At that point the
voltage measured 12 volts and was climbing all by itself.
The "setup" for charging a battery
With the acid added to the battery it started heating up. That is normal, so
don't worry about it. One "should" let it set for a couple of hours to cool off
before starting the charge. In this case it was late evening and I wanted it to
charge overnight and be ready for use in the morning. I decided to start
charging it at that time. Charging also heats up a battery. If I had started the
charge at the usual 10% of the amp hour rating of 12 amps, my charge of 1.2 amps
"could" have added enough heat to make it hotter than desired. Put your hand on
a battery that has been filled for 10-15 minutes, it is quite hot. You don't
ever want your charging to make it that hot. Now is where the Variac comes in. I
am using the homemade one shown above. I just adjusted the Variac to provide the
charger with just enough voltage to give me the desired charge current. I
decided to charge it at about 1/2 amp. That is a really low trickle, but
enough to charge it up. It is far better to charge a
battery slowly than at a fast rate.
This shows the output voltage of the Variac. I really have no interest in the
fact that it is 104 volts, only that I get the desired charging current. Within
one hour the charge voltage had risen to 12.48. In the morning I found that the
current had dropped to .3 amps and the charge voltage had risen to 13.89 shown
below. That could sound like it is not fully charged up, so I increased the
charge current to about .6 amps and that required a voltage of 14.2. That showed
me that the battery was fully charged. I arrived at the .6 amps by knowing
that if I had used the 10% rule to fully charge it, the current would drop to
about 1/2 of the starting current, or .6 amps.
What I am trying to say is that I wanted to safely charge the battery
overnight. I didn't want to risk overheating it, so I lowered the charge rate to
just less than 1/2 of the standard 10%. By morning it had fully charged.
My final charge voltage.
I have always gotten a kick out of the advertising on chargers saying that
they are engineered to automatically adjust the voltage down to meet the need. That is crap. They have no choice in the matter. As the voltage on the battery
goes up, the amount of difference is less and so is the current. They didn't
engineer it, they were limited by physics. A totally natural limit is
advertised as some "valuable thing" that only they have.
Now, recently, some new games are being played with chargers, but that is
recent and doesn't apply to the older simple transformer types. They might be
called "smart chargers" or "battery tenders" or some such. These automatically
stop charging when the battery reaches some predetermined voltage. When the
internal leakage drops the voltage down again, it starts charging again. That is
fine only if the charger is made for that type and size of battery. With
the current variety of batteries and types, we would need several of these
expensive chargers to fulfill the needs.
The amp hour rating is only when new. As the battery ages, it is losing its
rating. When it drops below the requirement, that is when it won't start. It
is up to you to control the requirement. An engine in good tune will require
less cranking. A starter with good bushings will deliver more energy with a
lower current draw. If your bike is in poor tune with, has a starter with
sloppy bushings and has an old battery, it can easily fail to start. Of
course the battery could have a dramatic failure, but in general they are on a
gradual downhill slide from new.
Batteries are dangerous
If one charges at too high (the original 10%) of rate, the battery can
overheat. The heat can warp the plates. The warping can cause them to break or
short out. The gas in the case is oxygen and hydrogen, or the two parts of
water. They are in the perfect ratio to ignite. They are contained
in the case and we call that a "bomb".
I recently found this picture of the exploded battery,. It happened in 1992
in my (ham shack) radio room. What you see in the center of the picture is the
exploded battery sitting in a plastic pan. The top is in another pan off to the
left. The battery top hit the ceiling so hard that it damaged it. I was using an
old car battery as a power supply for some radio equipment. I had failed to
reduce the charge current on an older battery. The amp hour capacity had
diminished to a small fraction of what it was when new, but I was still charging
10% of the "new rating". I was probably charging at 100% of the greatly
diminished rating. $6000 later I had learned a lesson. I am glad that we were
all downstairs when it blew up. It sounded like dynamite. Most of the damage was
from the acid. Printed circuits don't like acid dripping onto them.
Carpeting tends to fall apart when soaked with acid.
Typical email question
In an email question on one of the BMW motorcycle lists, the statement was
"My battery voltage was measured after my brief run and again this
morning. After the run it was 12.71 volts and this morning it was 12.72V. This
indicates to me that it is in the fully charged range of between 12.65 and 12.9. So in my opinion it is not a tired battery and supplies the amps it was rated
I think that electricity can be confusing and is often misunderstood. Here is
a layman's version (mine) of this issue.
Here is my analysis of what the two measurements of the earlier email
suggests. Since the first resting voltage reading of 12.71 is under the
recommended charge voltage of about 14, the test was made after the surface
charge had dissipated, that is good. It also suggests, but doesn't prove, that
the charge voltage is in the ballpark of 14, that is good. The second reading,
that is equal to the first, after some hours at rest, shows that the battery has
no drain from the bike wiring, and that is good. It also shows that it
doesn't have an internal short, and that is good.
We know nothing about the battery capacity. That means that we don't know
anything about it's condition because nothing important has been measured.
It now needs a load test.
If you really want some serious battery info and still in layman's terms,
If this article contradicts me, believe this excellent article, not me.
Measure the charge voltage
1. The charge voltage is the final voltage that is read at the battery
terminals while the engine is running. This is really a test of the charging
system only. It may take several minutes of charging at 2500 rpm to arrive at a
final voltage. It must be measured at the terminals, not at a lug that is
fastened to the terminal. True, the two should be the same, but sometimes
aren't. A lack of dialectric grease will allow oxygen into the connection and
corrosion is the result. Then a voltage drop can occur.
Most systems have a voltage regulator that can be adjusted. BMW's up to the
solid state can be adjusted and maybe those too. The /5 should read about 14.1
to 14.2 volts. Under 14.0 volts and the battery will be found to be
Important note; It is commonly accepted that the charge
voltage should show 13.8 volts. I went into the testing and servicing of
batteries with that number. We found that, for whatever reason, it wasn't enough
for the charge voltage measurement to show only 13.8, but just over 14 volts was
needed. Later I heard that Ford had the same higher numbers.
2. The resting voltage, is taken after the charge voltage is measured. This
is one of the most common voltages measured and the least informative. It varies
with battery condition. I must admit that I know little about this resting
voltage. From the standpoint of servicing the charging system, we couldn't waste
time on it. We set up and adjusted the charge voltage to what we knew would
work. Why go back later and check the resting voltage?
3. The voltage under load. This is the voltage measured while the battery is
under some stated load for a specified length of time. This is a real test of
the batteries capacity. This is frequently called a load test.
Sorry, but I don't have the specs for this one.
The problem with #2 is that one is probably measuring the surface charge. Any
lead acid battery will have a surface charge that can be misleading. We see
this, in the extreme, in our cellular telephone transmitter sites. We usually
have 4 thousand pounds of batteries for "back up" when the power fails. When the
power first fails, the equipment sounds ok for a few minutes and soon starts to
"wind down" in sound. After a few minutes the sound (fans) will stabilize. The
charge voltage of 27 volts causes the surface charge to also be 27 volts. As
soon as the surface charge starts to dissipate, the voltage gradually drops off
to 24 volts and will stay there for hours. A good set of batteries will keep us
on the air for 6-8 hours. After those hours the voltage starts to drop off and
soon it reaches 22 volts and then our protection cuts in to save the batteries
and we are off of the air. A worn out set of batteries may only run for less
than an hour at 24 volts and then fail. In both cases the charge voltage
and surface voltage showed that all is OK.
A battery can be so low in capacity and still show full voltage when it is
charged. It could be compared to a very small glass (6" high container) that is
full of water. It may be full, but has very little stored water. Compare that to
a huge reservoir that also is full at 6" deep, but holds thousands of gallons. Our volt meter is the same as the ruler that measured the depth of the water. In
both cases the depth is equal, so both containers are full. A battery can be
physically "large" and once had a full ability to store electricity, but it's
real capacity is diminished to almost nothing. It will still read full voltage. The battery is now a "small" one.
Our once "huge reservoir" is now really only a small glass full of water.
When the battery capacity drops significantly, say 90%, and we still charge
it at the rate for a new battery, we are over charging it. We all know to only
charge at 10% of capacity, but what is the capacity for a used battery? It the
battery is down to only 10% of it's original capacity, then it is being over
charged by 10 times. This makes the battery "hot." The heat can cause the plates
to warp. They may warp enough to short out. That is the condition
where they tend to explode.
If you have never seen a battery explode, then you are lucky indeed. The
power available is much larger than I would have expected. The damage or injury
can be great. An old battery is to be treated even more carefully than a new
one. It is a small stick of dynamite.
The new BMW motorcycle battery charger
It seems that BMW dealers are claiming that BMW has available some new high
tech battery charger that is necessary to charge the "so called" gel cell
batteries. I have no knowledge of the accuracy of this claim. This
is new in late 2003.
Jump starting a motorcycle from a car
At first it may seem that a car system is too big to jump a much smaller
motorcycle. If you had the largest 12 volt vehicle system in the world, it
would be OK to jump the smallest 12 volt system in the world. If the
voltages are nearly equal, then there is no problem with the theory. One
must protect from an explosion as mentioned above.
This is a post from Curt Henry on the Boxerworks forum about jump starting.
It is so well done that I just lifted it and pasted it here.
Being from the frozen north, we receive a set of jumper cables at birth
and are required to carry them to the grave. When hooking up jumper
cables ALWAYS CONNECT THE Positive TERMINAL FIRST TO THE positive TERMINALS ON
BOTH BATTERIES! THEN HOOK THE NEGATIVE CABLE TO A GOOD GROUND, BUT NOT THE
BATTERY!!!!!!! Do not SCREW UP this procedure, a arc could cause one of the
batteries to BLOW UP, bad, very bad.
After the jumped engine is running disconnect the NEGATIVE JUMPER FIRST
FROM THE FROM THE JUMPED VEHICLE, BEING CAREFUL NOT TO TOUCH ANY GROUND, BIG
ARC, ALSO VERY BAD. Then remove the positive cable and carefully store
your cables until the next time you need them.
Car starting story; When I was in school I had a gas powered car starting
unit in the back of a Dodge Power Wagon. This unit had 6-12-18 and 24 volt
outputs. After jumping myself to get the Dodge running in very cold weather, I
would drive the M of U campus looking for those poor souls who car would not
start. Stop and offer them a jump for $20, good money in those day. Hook up the
cable from the starting unit, 12 volt, a shot of starting fluid and most would
start right up. If 12 volts didn't do it, I would jump it up to 18 then 24, not
many would not start with spinning over that fast. I am sure that I fried
a few alternators and did not do the starter any good, but with luck I would
never see any of them again. Kept me in beer for the long dark winters here in