Battery testing, charging & jump starting

BMW motorcycle battery testing, removal, trickle charging, and jump-starting. 

by Duane Ausherman

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 ensure safe operation around a battery.  Here are a few tips.

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.

Sparks will fly if one removes the positive (ungrounded) cable and accidentally allows the tool to touch the ground.  If some circuit is drawing current, then a spark can occur when the cable is lifted away from the battery.  For that reason, it is best to first remove the grounded wire, which is usually the negative one.  It is always the negative one on a BMW motorcycle.  One should 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 turned off.  I like to first flash the two clamps 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 with 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 on the charger.  If your charger has a lower setting, always use it first to check for 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 mechanically 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 of boiling 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, the 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 it 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, it was upgraded to 120 X 90 X 165 mm high for the USA only.  

Battery removal, /5, /6, and /7 (1970-80)

Generally, the battery comes out of the top.  Don’t waste time removing the air cleaner and associated parts.  The two bolts holding the rear frame section onto the mainframe section may need to be loosened or removed for the larger battery on the LWB models.

Trickle charge your BMW battery.

The only difference between a trickle charger and a BIG ASS CHARGER is its charge voltage while under load.  With a higher charge voltage, the battery will draw a higher current.  The charger’s ability to deliver that larger current makes it a BIG ASS CHARGER.  A charger that has a high voltage, but no capacity to deliver the current, quickly has its voltage drawn down to a smaller amount and little current.

The enemy of charging is heat.  Go back in an hour and put your hand on 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, you know it was overcharged.  A slower charge is safer and will increase battery life too.

The normal charge rate is 10%.  A typical BMW battery of 20 amp hour rating 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, 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 is no longer a 20 amp battery.  The capacity starts 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 unknown capacity, and it is better to charge it at less than the original amount.

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 sit 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, and it will be 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.  

Fake marketing

I have always gotten a kick out of the advertising on chargers saying that they are engineered to adjust the voltage down to meet the need automatically.  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.  The laws of Physics limited them.  A natural limit is advertised as some “valuable thing” that only they have.

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 only applies when new.  As the battery ages, it loses 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, 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 the time that they are new.

Batteries are dangerous

If you charge the battery at too high (the original 10%) 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.”

This shows the battery after it exploded in the house. I was still charging it at 10% of its full rating.

I recently found this picture of an 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 partially seen 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 failed to reduce the charge current enough 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 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 made;

“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, I think it is not a tired battery and supplies the amps it was rated for.”

My response;

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 suggest.  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, which is good.  It also suggests but doesn’t prove that the charge voltage is in the ballpark of 14, which 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 its condition because nothing important has been measured.  It now needs what is called a load test.

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 battery 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 dielectric grease will allow oxygen into the connection, resulting in corrosion.  Then a voltage drop can occur and will give you a false reading.

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 undercharged.

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 were needed.  Later I learned that Ford had the same higher numbers.   

2.  The resting voltage is taken after the charge voltage is measured.  Don’t waste your time on it.

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 battery’s capacity.  This is called a load test.

The problem with #2 is that it probably measures the surface charge.  Any lead-acid battery will have a surface charge that can be misleading.

A battery can be low in capacity and still show full voltage when it is charged.  It could be compared to a tiny 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 voltmeter 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 the full ability to store electricity, but its 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 overcharging it.  We all know that we should only charge at 10% of capacity.  What is the capacity of a used battery?  If the battery is down to only 10% of its original capacity, then it is overcharged by ten 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, you are lucky.  The power available is much larger than I would have expected.  The damage or injury can be significant.  An old battery is to be treated even more carefully than a new one.  It is a small stick of dynamite, as shown when my battery exploded, as shown above.

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 do not know the accuracy of this 2003 claim.

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 in theory.  One must protect from an explosion, as mentioned above.

Here is a post from Curt Henry on the Boxerworks forum about jump-starting.  Curt did so well 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 because an 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 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.

Grease the terminals on your BMW motorcycle battery, coil connections, and other items

This info applies to all models of BMW.

The comment was made that the grease for battery terminals has some resistance.  It is an insulator, so it seems to make sense.  That easily hides the fact that it still should be used between the terminals.

In my 17 years working as an inspector of telephone cell site construction, I dealt with large power supplies and batteries.  We have an electrical current available of a few thousand amps from the battery bank.  That is ten times that of an electric welder.  A tool that accidentally crosses it can just about vaporize.  It is not dangerous as far as getting shocked, as it is only 24 volts, but the danger is from getting burned.

The wires that connect them are as thick as your finger.  It takes special procedures and tools to attach the terminals to the wire and then bolt it down.  We grease everything.  The grease gets pressed out of between the connections and forms an airtight seal to keep air out.  Air contains humidity, which is water.  Water causes corrosion and increases the resistance, reducing current flow and becoming a diode that can cause radio interference.  As an inspector, I was required to see a bead of grease squeezed out and around the connection to prove that grease was used.

These are properly greased terminals in a cellular telephone (radio station) site. The key is for size reference. See the bead of light brown grease that has squeezed out and around the terminals? If you look closely you can also see the upper part of the terminal where the black cable goes into the terminal. Look just below the crimp and you can see a "window" or hole in the terminal. You can see some grease squeezed out through the hole. That is for me, the inspector, to be able to determined that the cable was fully greased before the terminal was crimped. All copper wire should be greased before the terminal is crimped.

These are properly greased terminals in a cellular telephone (radio station) site.  The key is for size reference.  See the bead of light brown grease that has squeezed out and around the terminals?  If you look closely, you can also see the upper part of the terminal where the black cable goes into the terminal.  Look just below the crimp, and you can see a “window” or hole in the terminal.  You can see some grease squeezed out through the hole.  That is for me, the inspector, to be able to determine that the cable was fully greased before the terminal was crimped.  All copper wire should be greased before the terminal is crimped.

How do we use this knowledge on our BMW motorcycle batteries?

This is evidence of what the “big boys” do about protecting electrical connections that carry large amounts of current.  Using this practice on all of your lead-acid batteries will help you.  If you don’t have the “official” grease, then use Vasoline.  Regular wheel-bearing grease isn’t ideal, but far better than nothing at all.

I suspect that we have all seen this under the hood of a car. This photo shows a corroded battery terminal. Grease won't prevent the leak, but it may prevent the same corrosion from deteriorating the connections.

I suspect that we have all seen this under the hood of a car.  Grease won’t prevent the leak, but it may prevent the same corrosion from deteriorating the connections.  The molded battery cable terminals are the best ones, but sometimes we are stuck with using a “universal” cable connector.  If it is the crimp type, then it is best if you solder it rather than crimp it.  If soldering isn’t an option, then use grease liberally on the wire before crimping.  At that point, it is best if you further protect the terminal by using some shrink tubing on the connection to help keep acid or water out.

Use a small spot of grease on the connector of any item with a battery, even a dry cell type.  How many times have you opened up a flashlight, multimeter, radio, or other appliance that is battery operated and found a leaky battery?  Usually, it erodes the metal terminal and can even eat it up.  The grease won’t prevent a battery from leaking, but it will save the metal terminal.

The ignition coil connections need to have grease under those push-on connectors too.  They cause a lot of trouble.  If all the push-on connectors had grease under them from the factory, you wouldn’t have a problem now.

A squirt of new “WD 40 with Silicone” or other lube in the handlebar switches once a year, and you will reduce that corrosion too.  They fail from the rainwater.

For charging system diagnosis, go to the well-written article by Buchanan.

As some of you may remember, I was a BMW motorcycle dealer and, therefore, also in the repair business.  Here is one thing that I learned.

A car battery costs just over a dollar a month to own.  A 60-month battery can be purchased for around $60-$80 on sale.  When you get near the battery life, it pays to replace it before it fails.  Of course, we all have a story where a battery outlasted the specified life by a year, so look at the bargain.  Those stories are nice, but far more are of a battery that didn’t quite make it to 60 months.  What is the worst-case scenario? I would say that a battery fails at midnight, in a bad neighborhood, in the rain, and you are way out where there is no cell service for calling AAA.

Is the small amount that you saved by keeping the battery to the end worth it?  Nearly everyone in that situation would say, “No way.”  Sacrifice those few months by replacing your working battery ahead of failure.

Updated 30 March 2023