Battery Tech Information

Battery Tech Tips

Charging and discharging

The very basic principle of batteries is that they discharge to provide electrical power, and then recharge later on. In a vehicle, this is based on the fact that the battery has a 'natural voltage' of about 12.8 volts.  This means that a battery that is not being charged or discharged will tend to have about 12.8 volts between the two terminals. This is usually referred to as the OCV, or Open-Circuit Voltage. Things get a bit more complicated when the battery is being charged or discharged; the actual voltage will be different from the OCV because it is no longer open-circuit.  At these times, the theoretical OCV is referred to as the EMF, or Electromotive Force. The difference between the EMF and the battery's actual voltage represents the amount of work it's doing (in discharge) or the amount of recharge it's accepting.

  1. Batteries discharge (provide current) when the battery's EMF is higher than the electrical system's voltage, and charge (accept current) when it is less. This reaction is fundamental.  Electrical loads act like short-circuits in a way, so they try to get to zero voltage.  The battery wants to be at 12.8, so it provides current to try to achieve a balance.  When the alternator is functioning, the system wants to be at 14.0 volts (typically) but the battery still only wants to be at 12.8, so it accepts current for the same reason.
  2. While electrical loads drive the system voltage down, the charging system tries to supply enough current to keep the voltage at a certain level. If the loads win, the battery discharges to try to make up the difference. This happens when starting, when running accessories (even the clock!) with the engine off, and when the bike is running but not charging enough. If the charging system (or an external charger) wins, the load is met and slightly exceeded, so that the battery is being slightly charged.
  3. Depending on the nature of your riding and the charging system of the bike, the battery may be alternately charging and discharging from time to time. This is called "load leveling", when the battery "kicks in" some power when needed, and takes it back when there's some to spare. This is OK as long as: a) more energy (at least 10% more) is returned to the battery than is removed, and b) the battery never becomes "empty". Starting is the most obvious drain on the battery, but cooling fans, electric clothing, lights and the like are all heavy system drains which might dig into the battery reserve at times (like idling at a traffic light). As long as the power is replaced in time you'll be OK, although these "deep cycles" will shorten your battery's life.
  4. Batteries should be charging during normal bike use. The battery's primary purpose is to provide current when the bike's electrical system demand is more than the charging system output, as described above. Ideally, though, this would only happen during starting; after that, the higher voltage maintained by the charging system will recharge the battery and then "float" it at a slightly-higher-than-EMF voltage. The battery would no longer be supplying power, since all demands would be met by the charging system of the bike. In reality, this does not happen on many older bikes; load-leveling (as described above) does.
  5. Although the loads may briefly overpower the charging system with no ill effect, continuous discharge during normal bike use is obviously undesirable. If this happens, the battery will probably never become fully charged unless an external charger is used when the bike is parked. However, in unusual circumstances this may be the only way to operate the bike. If the electrical load routinely exceeds the ability of the charging system, it can be ridden as long as the battery is recharged before the battery is fully discharged. Without eventual recharging the bike will not run.


Plate chemistry      

A 12V battery is made of six 2V cells in series. Each cell has positive and negative plates, with insulating "separators" between them. The plates are made by filling a lead gridwork with active material. The positive active material is lead dioxide, or PbO2, and the negative is "sponge lead," a porous lead mass providing lots of internal surface area.

  1. Simplification: H2SO4 (sulfuric acid) and water form an ionic solution, made of SO4 and H2 ions. When they meet the battery plates, the SO4 replaces the O2 on the positive (PbO2) plate, and they also replace an electron on the negative (Pb) plate, forming PbSO4 on each. The displaced electrons from the negative are the whole point to the battery's existence. The end result is an excess of electrons at the negative plate and thus a voltage with respect to the positive plate. The battery tries to achieve a chemical equilibrium where the degree of ionization balances the concentration of acid in the electrolyte (battery acid, or the mixture of sulfuric acid and water in the battery). If it is "pushed" to a higher or lower voltage (i.e.; a higher or lower concentration of negative ions) the reaction will proceed in the correct direction to re-approach equilibrium (it will charge or discharge).
  2. If the electrons are allowed to travel (via an external load like a starter) to the positive plate, the SO4 from the acid replaces the O2 in the positive (PbO2) plate, forming PbSO4 (lead sulfate). The same thing happens on the negative plate, and an electron is replaced. The loss of the SO4 weakens the electrolyte. The remaining H2 ions in the electrolyte combine with the O2 ions, which were displaced from the positive plate and form water, diluting the electrolyte further. Equilibrium will now occur at a lower concentration of ions, due to the lower concentration of H2SO4. You see this as a partially discharged battery having a lower voltage. When either plate has no more material that can be converted, or the acid is fully diluted, the battery is fully discharged. Note: If the battery is left discharged too long, the lead sulfate will form large, hard crystals on the plates and will not be able to be forced back into the acid. These crystals are large enough to physically clog the pores in the plate surface. The whitish appearance of plates is this permanent PbSO4, which seals the plate surface off from the electrolyte, rendering it useless.
  3. During charging, the higher external voltage (from the charging system) forces electrons into the battery in the opposite direction, reversing the reaction. After all the SO4s have been forced back into the acid (battery is fully charged now), any further current electrolyzes the water in the electrolyte into hydrogen and oxygen, visible as bubbles rising to the surface. Batteries are not really fully charged until this happens. (Sealed batteries differ in this regard).

    Caution! The gas is a very explosive mixture!

Performance, and how it affects battery selection

Two typical measures of battery performance are capacity and cranking current.

  1. Capacity is determined by a slow discharge (usually over 10 hours) until the battery reaches 10.5 volts. The test current multiplied by the discharge time is the capacity in Amp-hours (AH). This is proportional to the volume of the plates and acid, i.e., the battery's physical size. The relatively long test time allows diffusion to replenish the weakened electrolyte in and near the plates, ensuring that most of the sulfates in the acid can be used.
  2. Cranking performance is usually measured in Cold Cranking Amps. CCA is the highest current the battery can supply at 0° F for 30 seconds without dropping below 7.2 volts. Due to the short time and the low electrolyte temperature, diffusion is negligible and only the acid in the plate surface can be replenished at all. The weakened acid inside the plates cannot be replenished in such a short time, so battery CCA is directly proportional to the total plate surface area, the negative in particular. The eventual replenishment of the weakened acid is what allows the battery to "recover" when you pause after prolonged cranking. Keep in mind, actual cranking power increases with temperature; the CCA number only applies to 0° F. Sometimes you may see a reference to "CA", or Cranking Amps. This is the same test, performed at 32° F.
  3. Batteryselection, then, depends on how much cranking power and capacity you want. Larger batteries generally have larger plates, providing both increased volume (capacity) and surface area (cranking power).
    1. You need a good CCA "safety factor" for very cold weather and hard starts. Hard starts can include poor state of tune (long cranking required), thick oil, high compression, and so on. With a new K75, I require very little CCA to start, even in winter (the stock battery had less than 180) but my conditions are pretty optimal.
    2. You need good AH capacity if you have any load-leveling concerns as mentioned previously (periods of time where the loads outweigh the charging). On the K, with a 700W alternator I have no load-leveling concerns since the electrical system is always meeting the demands. I use the smallest battery possible, since I don't want to carry the extra weight. A larger size would not hurt me, though.
  4. Battery life is another concern. The cycling action weakens the positive plate over time, causing it to slowly shed particles of the active material. Larger batteries will be used to a lower "depth of discharge," causing less of the weakening. Furthermore, the loss of that material will be less significant since there was more to start with. In practice, these effects are probably small, unless you run with a lot of "load leveling".

Care & feeding

Simply keeping the electrolyte levels up and keeping it charged will usually enable a battery to last for years.

  1. Batteries will gradually lose their charge when not in use, due to small currents flowing within each cell. This self-discharge causes the battery to lose between
    percent and 25 percent of its charge in a month. The use of lead/calcium alloys decreases the self-discharge, as does a lower ambient temperature. Remember, though, discharged batteries can freeze at temperatures slightly under the freezing point of water. Cold is good for storage, but the battery must also be recharged. DON'T OVERCHARGE! Charging 5 AH per month is enough for all but the largest motorcycle batteries during the cold off-season. You'll get this overnight with most chargers.
  2. Due to the charging, both on and off the bike, the electrolyte loses water through the electrolysis mentioned previously. Keeping the level above the plate tops by adding distilled water will prevent plate damage Distilled water is used because mineral impurities in tap water can increase the self-discharge rate dramatically.
  3. If it's dirty, clean it. The water content of spilled electrolyte will evaporate, leaving concentrated sulfuric acid. This can provide a conductive path on the battery's cover, discharging the battery as the bike sits. Furthermore, it does not help the battery tray or hold-downs. If the top looks oily, take the opportunity to remove the battery and wash it with soap and water. Check the electrolyte level, and clean the connections while you're at it. A bit of terminal coating (spray, or even Vaseline) is a nice touch, once it's all back together.



  1. My battery won't hold its charge. What's wrong?
    1. Did you maintain it over the winter? Natural self-discharge may have left lead sulfate crystals on the plates too long. If they can no longer be converted back into acid, the plate surface has become "sealed off," and the electrons cannot pass through it. A sulfated battery can appear charged, but only the outside layer of the plates is active. This surface charge will dissipate quickly under load; a hydrometer will show the electrolyte to be very weak. Also, if a battery is completely discharged, some lead will dissolve in the electrolyte (now just water). But as soon as you charge it, the lead will come back out of solution. This can form a conductive path through a separator, preventing it from staying charged. Replacement time!
    2. It may be damaged from overcharging, either on the bike or on a charger. This softens the positive active material, causing it to lose electrical contact with the grid. Also, as it falls off the grid it piles up on the bottom. When it piles high enough the plates will be shorted out and then the cell will never stay charged. Check the charging system, the battery charger, or your use of the battery charger if this happens. Vibration and age also cause this. When charging regularly, check the electrolyte level; it's easier than you think to dry a battery out.
    3. Your bike might have a current leak. Hook an ammeter in series with the battery (use the negative lead, it is easier) on the bike with the key off. Any significant reading (.005A or more) can give you a problem. .005A for 2 months is 7.2AH, a significant amount of your battery's capacity to be lost in 2 months. The clock + radio + alarm can easily exceed this, so your off-season maintenance must accommodate the current drain.
    4. There may be an internal defect. Measure the voltage as you try to start the bike. If it drops very low (like less than a volt) while you try to start, then returns immediately to normal voltage when you stop, there may be a broken internal connection. The battery is charged, but cannot deliver the current. Another internal defect is a short, caused by loose active material (see above) or plates contacting through or around a separator. This will quickly discharge that cell, leaving a 10V battery. More importantly, the plates in that cell will sulfate, preventing them from transmitting the current generated by the other cells.
  2. Why is the voltage so low?
    1. The battery voltage depends primarily on the acid strength, and to a lesser degree on temperature and design. As the state of charge increases, the acid becomes stronger. Fully charged, a flooded (typical) battery's acid specific gravity is normally 1.265 to 1.295 and the battery voltage (no-load) will be about 10 times that; 12.65 to 12.95. Low voltage, therefore, is generally caused by insufficient charging, or permanent sulfation (which lowers the concentration of the acid, since the sulfates are tied up elsewhere). Excessively high specific gravity (stronger acid) can cause excessive grid corrosion. Furthermore, it can raise the EMF high enough that the battery will not properly charge; i.e., the charging system voltage may not exceed the EMF enough to fully charge the battery in the time allotted.
  3. What's the difference between Calcium batteries and "regular" ones?
    1. Batteries with plates made from a Lead/Calcium alloy do not electrolyze as much water on charge, so less must be added. The disadvantage is that the grids do not tolerate repeated deep-cycling, where the battery is heavily discharged, then recharged. Normally, these batteries will only see very shallow discharges, so this is not a problem. "Regular" batteries have grids made of a lead/antimony alloy. They will stand more deep-cycling, but will consume more water.
  4. Should I get a bigger battery?
    1. As mentioned, with a large enough alternator (which begins charging at low enough engine speeds) the battery only needs to be big enough to start the bike reliably; the 18AH size is plenty for that under normal conditions. You only need to go larger if you drain a lot of current during non-charging periods. Many 1,000cc and larger bikes today use 10AH and 12AH batteries with less than 200 CCA. This saves about five pounds. If the weight doesn't bother you, larger isn't worse. The larger battery may also provide longer intervals between replacements since it can deteriorate more and still perform well.
    2. Remember, though, it's really only better if it does something that the smaller battery won't. Running the radio or electric vest with the engine off is an excellent rationale for using a larger battery. So is spending a lot of time at low RPM on bikes with weak charging systems; the more "load leveling" you need, the greater the AH capacity should be.
    3. Keep in mind: the energy drawn from the battery must be replaced, plus ~30 percent (to account for inefficiency). If the load exceeds the maximum alternator output, the battery will make up the difference until it is fully discharged. Then you stop.
  5. How much load can I put on the battery?
    1. This is tough. It depends on how long you want the battery to last, before becoming fully discharged or going to too low a voltage. If the load is hundreds of amps, the battery will only last seconds or minutes. If the load is fractions of an amp, it will last for hours and hours. Vehicle loads are supposed to be supplied by the alternator, not the battery. As noted earlier, the battery only has to supply a total of 1-2 AH, max, if the engine starts and runs normally. To evaluate load-leveling concerns, convert the load (usually Watts) into Amps (divide by twelve) and count AH out and AH in.

Example 1: 18AH battery, no charging below 3,000 RPM, electric vest & lights. Running for 30 minutes before getting up to speed.



Lights (120W / 12) * 0.5 hours


Vest (33W / 12) * 0.5 hours


Ignition (just a guess, 36W)


total AH


Battery is sufficient, but the bike may not start well if the engine stalls at this point...

Example 2: 18AH battery, no charging below 3,000 RPM, radio, electric suit, 100 Watt each fog and driving lights plus normal lights. Running for 30 minutes before getting up to speed.



Lights (120W / 12) * .5 hours


Radio 24W


Fog and/or driving (100W/12) * .5 hours


Ignition (36W/12) * .5 hours


Suit (100W / 12) * .5 hours


total AH


Battery is too small!

The larger battery would be good here! Note that the bike may keep running. The system voltage will drift down as the battery discharges, so the current drawn will drop somewhat, "extending" the run time. Depending on the ignition system, the plugs may keep firing at the reduced voltage. Fuel injection might become decidedly unhappy. So, it may run longer, but that also means the bike must be operated in the "charging" mode longer to replenish the charge in the battery.

These figures use incredibly simplified numbers and assumptions, but can provide general guidance.

 Courtesy of John Eng, Nick’s BMW service manager, VJEMC officer