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Battery Filling and Charging

How To Fill and Charge a New Battery

By Robert Fleischer

Editor’s Note: This item was originally sent to the Airheads mailing list. It is posted here with the permission of the author.

From: Robert Fleischer <>
Date: Tue, 2 Mar 1999 09:38:57 -0800

>Based on comments recently posted to this list …
>buy a dry battery, add acid and allow to sit (over night?)
>then charge slowly. Yes?

I am posting this in depth, so that all ‘Heads out there who have limited experience with lead-acid batteries will know what to expect and how to put a new battery into service. I hope this information is useful. I cannot count how many times I have dealt with Beemer-folks over the years, with electrical problems, and failed batteries seem to be a real common problem, particularly bad when you are miles from the nearest (MOA) anonymous K & K place.

Have to start this someplace, so let’s begin with the battery purchase. If you purchase a battery, add acid, and put it into service immediately, you WILL NOT get good service life and performance over the years from that battery!!

Some dealers still do this … and I don’t really blame them too much, unless they sell lots of batteries, and therefore should be having one or more all done properly and on a trickle charger. Remember, it is that dealer who has been asked to get you back on the road TODAY … like RIGHT NOW!

I will assume you are going to purchase a brand-new NON-sealed lead-acid battery. NON-sealed herein means you have to add acid to activate the battery.

These are shipped ‘dry’, that is, withOUT acid, for a reason: deterioration begins the instant you add the acid. Remove the plastic packaging, if any, set the battery in a safe place in a glass dish of some sort. The battery likely has an overflow and gas venting tube connection; be sure it is open. If the battery came with a plastic 90 degree angle adapter for this gas port, install it and the drip tube now. Put on safety goggles, and use whatever method is appropriate for the sulphuric-acid battery mixture that came with your battery, to enable filling. I suggest a kid’s toy plastic funnel, etc. If your battery did not come with an acid container, go to your local auto supply store and obtain enough. Fill the battery to someplace between the lower and upper level limit lines. If you spill any acid, wipe it up immediately and dispose of the ‘wipe’ right away. This stuff is dangerous and highly corrosive. Leave the cell covers on very loosely. Let the battery sit for an hour or so. Then rock the battery back and forth and tap it gently on your workbench. A few times is enough. The idea is to work some of the gas bubbles to the surface. Refill the battery to the upper level for all cells. It is normal for the battery to get warm. Some folks like to let the battery sit overnight at this point. I have no argument with that, but let it sit another hour or three, at the least. You can now begin the charging process.

Regarding the question of what Specific Gravity the acid should be, I’m not in my shop where I have the three grades used for batteries listed … two are pretty common. You can use any commercial battery acid generally, but things work out best using the one specified on the paperwork shipped with your new battery. The actual concentration in use tends to equalize out after considerable miles and charging.

BIG DISCLAIMER: I am not responsible for your damaged eyes if you fail to use goggles … in particular if you ignite the hydrogen gas given off by batteries during (especially) the charging process!! Keep those sparks away from the battery as best you can by making solid battery charger connections and using the charger power plug to turn power on and off, not removing a battery lead! Keep the cell covers in place. Let’s be careful out there!

You can use a Battery Tender, a very good charger, or any other type of charger, but you do NOT want to charge at a rate higher than 10% of the ampere-hour rating of your battery. For our BMW’s, this generally means a maximum of approximately 3 amperes. Charging at a rate approaching 3 amperes is not a great idea however. I prefer 1 to 2 amperes rated chargers. Charge the battery about 16 hours or so; charging up to 24 hours on a 1 or 2 ampere rated charger won’t hurt anything. If you are monitoring the battery voltage with an ACCURATE voltmeter, the voltage will eventually rise close to 14 volts, give or take a bit. If you have a 6 volt BMW, all voltage readings in this posting should be divided by 2.0. Battery chargers vary quite a bit, particularly the simpler unregulated types. At room temperature, you only need about 13.8 to 14.25 volts for a full charge. Depending on the charger size and rate of actual charging, the battery may be bubbling (this is called ‘gassing’). This is normal, and the cells should eventually all be gassing about the same. It is rare, but if one cell doesn’t, and the others do, that cell is bad.

After charging fully, disconnect the charger, do the rocking/tapping again, and let the battery sit for a few hours, not just minutes. Top up the battery with acid, to the upper level line. You will never be adding acid, only distilled water, to this battery again.

When you install the battery, do not forget to clean the cable connections, coat the positive (+) cable connection with red goop (NCP2 or similar anti-corrosion stuff from your auto parts store … use an acid brush with this stuff). When installing your negative lead, cut out a small portion portion of the connection spade lug, just enough so it can slip over the speedometer drive screw threads. You will find this a useful modification. DO NOT install a switch instead, they have, or will, develop resistance (I don’t care WHAT the sellers say!).

DO NOT over tighten this speedometer drive screw! Check the hole in the screw … it is a vent!

Depending on the size of the charger, and its manufacturer’s design, after a full charge, and while still charging, you will have at least 13.8 volts measured at the battery terminals. Once the battery is disconnected from the charger, the terminal voltage will begin to decrease, relatively rapidly, and will settle in at a lower value after a few hours. The settle-in voltage, assuming no load other than perhaps the clock on your bike, is likely to be around 12 volts.

If you turn your lights on, this voltage will drop. Assuming a freshly charged battery, if the voltage drops to AROUND 10.5 or 11 with the lights turned on for a minute, the battery is likely shot and needs replacement. A very much larger load is placed on the battery during cranking. If this voltage falls below 10.5 or so, you have a bad battery or bad starter motor. In all cases I am assuming you are measuring at the battery lead metal posts themselves, and not the bikes voltmeter, which generally read 1/2 volt or more low, particularly when the bikes wiring connections and fuse connections are not clean and shiny bright and solid.

BMW has used several versions of its basic alternators over the years. Ratings of 180, 240, 280 … etc. watts. There are differences in the alternator rotors, stators, diode boards, and connections. They all work OK when in good condition.

This is a posting about batteries and charging, not alternator circuits, so I won’t go into them this time. Assuming your alternator is working correctly, and that you are not overloading with too many lights and heated vests and such, your voltage, on the road, and at 4000 rpm (an engine speed that is USUALLY adequate for full alternator output, no matter the load), will be around 13.8 volts, APPROXIMATELY!

If you have a BMW factory voltmeter on your bike, they are generally accurate, but only read the voltage at their terminals, not the somewhat higher level at the battery. BTW, the motorcycle voltmeter guts are NOT the same as the BMW car voltmeter guts. Motorcycle voltmeters have a highly damped mechanism, so the needles don’t bounce around a lot.

Maintaining your battery: In many if not nearly all BMW motorcycles, the battery sits at an angle when you are off the bike and trying to see the water level. I mean the center stand is in use, not the side stand! Typically the water level lines are forward facing, the battery is tilted with the front upwards.

I suggest you just check that your battery plates are covered by about 1/4 inch or so. If you are installing that new battery with the level set to the top line while sitting on your workbench, once installed, check the level, and note the difference. Use only distilled water. Water containing minerals WILL reduce your battery’s life. Battery life depends on maintenance. Maintenance includes keeping the battery charged and the water level adequate. Some folks use a Battery Tender or similar, plugging it in after every ride, and leaving it on until the next ride. For most of us, unless you do a lot of in-town stop and go riding and have problems keeping the battery fully charged, simply using any motorcycle charger once a month, overnight, is likely enough. As long as the plates look covered by liquid, you have enough. If your voltage regulator on your bike is set to over 14.0 volts (over 14.45 is a bad idea), you will ‘use’ water somewhat faster, and for long trips, check for about 1/4 inch over the plates … particularly if in hot weather.

Battery voltage requirements for charging vary with temperature. Some on-bike voltage regulators do not have a great temperature compensating circuit. Even the old Bosch mechanical regulators had temperature regulation, and all are supposed to, some are better than others. Expect the voltage, assuming a fully charged battery, to be higher when the regulator is cold. It takes more rpm’s than most folks think, in order for the alternator to put out enough to take care of the bike’s ignition, lights, etc., let alone have enough to recharge and keep charged, the battery, from starting, etc. Don’t expect to see much, if any, charging under 3000 RPM or so.

If anyone wants more on this subject, I will be happy to post same, if I have the knowledge … or; let others add to this.


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