Selecting batteries for a RV's portable electronic gear.



DISCLAIMER

Many RV'ers carry portable electronics on board, such as portable radios, handheld trail GPS units, cameras, or other items. As these devices typically use AA size batteries, there are some considerations as to what ones to use.

One trip to a well-stocked department store will leave you scratching your head trying to determine which battery type is best. There are 4 popular battery types that are of interest to the typical RVer, they are:


Figure 1. Battery comparison.

While there are other battery types available (rechargeable Alkaline, Heavy-Duty Zinc), they are not as common as the types mentioned, and may require specialized charging equipment that may be expensive. They offer no real advantages to the battery types mentioned. Heavy duty Carbon-Zinc cells have all but gone out of favor. While they are the cheapest non-rechargeable AA batteries you can purchase, their capacity is only a few hundred mAH in the AA size. While initially less expensive than even Alkaline, their capacity is perhaps only 25%, which results in a more costly battery.

 

 AlkalineNi-MHNi-CadLithium
Voltage1.5V1.25V1.25v1.5V (1.7V open ckt)
Typical Capacity2400mAH1900~2700mAH1000mAH>3000mAH
RechargeabilityNone1000~1500 cycles100-300 cyclesNone
Shelf Life5~7 yr2~5 yr (per charge)1~3 mo (per charge)15 yr
Cost per Battery$1.25$3.00$3.00 $1.75
Cost per Use$1.25< 1¢ $1.75

Figure 2. Typical AA battery characteristics (of various manufacturers)

Battery Terminology:


Voodoo Charging

Incorrectly charging a battery is perhaps the main cause of poor performance and premature battery failure.

To better understand charging, I can offer an analogy that might make more sense. Think of charging as boiling a pot of water. In this scenario, the temperature of the water is the amount of charge in the battery, with the boiling point equivalent to full charge. And the amount of heat applied is the charging current.

As you apply heat to the water, it's temperature begins to raise, and when it gets to the boiling point, the "battery" is fully charged, and the water begins to boil. Assuming the pot is covered, if the heat source is low enough, evaporation of the water in the form of steam will not occur, and the bubbles will be dissipated back into the water. However, if the heat source is too high, boiling continues to occur, and soon pressure will raise under the cover, and eventually steam will escape.

It should be noted that you can use a high heat to get the water to the boiling point (rapid charge), but then the heat must be reduced to maintain a state of equilibrium. This low heat source in a battery is known as a trickle charge.

In a rechargeable battery regardless of chemistry, the same thing occurs. You can rapid charge most batteries up to the point where they become fully charged. During this phase, a chemical change is occuring which returns molecules back into the battery plates that were released into the electrolyte. The particular molecules depend on the battery chemistry.


Safety vents.

When the battery reaches full charge, no more chemical change is possible, and the electrolyte begins to gas (or boil as in the analogy). If the charge rate is low enough, the gas is dissipated back into the electrolyte and equilibrium is achieved. This is known as a trickle charge.

However, if the charge rate is too high, the battery will gas, it's internal temperature and pressure will raise, and eventually it will blow the safety vent typically located under the positive terminal. For most AA batteries, the vent is not re-sealable, and once opened, the electrolyte can leak out or dry out. At this point, the battery is ruined.

General Electric did make some re-sealable vent NiCad batteries in the '80s, but this is a rare exception.

Slow Chargers: Also known as trickle chargers, overnight chargers, or 16 hour chargers, these chargers supply a low charge rate to the battery that will completely charge the battery over time, and when the battery goes into overcharge, the charge rate is low enough as to be safe. A battery can remain under charge in these chargers almost indefinately. NiMH batteries are especially susceptable to overcharging, and can easily be damaged if the charge rate is too high when the battery is overcharged.


Don't mix "kit" fast chargers and battery brands.


Figure 3. Charge Rates

Fast Chargers: Fast (or Rapid) chargers typically charge the battery in one, two, or four hours, and use a charge rate high enough that it will damage the battery should the charge remain applied when the battery goes into overcharge. For the most part, the cheaper versions of these chargers cannot fully charge a battery. Convenience is swapped for capacity.

The cheapest fast chargers use a simple timed charge. With these chargers, there is a safety margin built in, and high rate of charge is cut-off when the battery is perhaps 75% fully charged so as to prevent any chance of overcharging. Batteries to be charged with these chargers must be fully discharged. Recharging a partially discharged battery can result in overcharging at a high charge rate, which will likely damage the battery.

Smart chargers are also fast chargers, but with circuitry to detect voltage, temperture, or some combination of the two, to determine when the battery is nearing the full charge state. When this point is reached, the charging current is removed. As with their cheaper fast charger brethren, the battery is not quite fully charged. However, the best smart chargers will change to "trickle mode" and apply a low charge rate to the battery which will top it off allowing the battery to achieve full charge. Like a slow charger, this low charge can remain on the battery without damage.

Charger Brands: It is common practice to charge Brand A batteries in Brand B charger. Battery manufacturer chargers; those that come with batteries in a "kit" are the most likely to cause problems. Cross-branding batteries with chargers is not a very good idea, at least for some FAST chargers. As each manufacturer's batteries have specific formulations and capacities, the algorithms used for determining when a battery is fully charged may differ from brand to brand.

Charging batteries in the wrong charger can damage them. In fact, you will usually void the manufacturer's warranty by charging a battery in a different brand charger. This not because of liability issues, it is because the battery can be damaged.


Only charge Sanyo Eneloop batteries in this charger.

Please note that this caution is primarily intended for the typical manufacturer-made charger supplied with batteries. Using a purpose-built Fast charger for, one that is designed to charge batteries from any manufacturer are generally safe.

As well, a slow charger (even those that come with batteries) can generally be used to charge any brand battery as long as the charger and battery chemistry is the same (i.e. using a NiMH charger for NiMH batteries), as there are typically no voltage/temperature circuity requirements to determine when the battery goes into overcharge. Slow chargers typically provide a trickle current that is 0.1C (or less) to the battery.


Maha PowerEx MH-C9000 Charger/Analyzer

Bank Chargers: Found in some "kit" chargers, these inexpensive chargers electrically connect adjacent battery slots together, so that you have to recharge two batteries at a time. Some rapid chargers are of this type as well, which can complicate the overcharge sensing circuity - another good reason to avoid using different brand batteries in these chargers. Avoid combining different battery types and chemistrys in these chargers. For example, you should never charge a AA and AAA battery at the same time in these chargers. Their capacities differ significantly, and the AAA battery will go into overcharge long before the AA battery will.

This restriction should even include avoiding charging batteries of the same size and type, but different capacities. For example, do not charge a 2,700mAH PowerEx battery in the same bank as a Sanyo 1,950mAH battery.

Higher-priced (non-banking) chargers usually allow battery types and sizes to be mixed. But always read the manufacturer manual to be sure.

Charger/Analyzers: A special breed of high end chargers, called Charger/Analyzers, such as the PowerEx/Maha C-9000 do an excellent job of properly charging Ni-MH and Ni-Cad batteries from any manufacturer. This charger typically employs sophisticated algorithms to measure voltage and temperature of the battery to determine when the charge current should be removed.

Recharging Non-Rechargeable Batteries: Some products exist on the marketplace that allow you to recharge non-rechargeable Alkaline batteries. These devices are sometimes called "battery-rejuvenators", "battery-recyclers" and the like. I don't use them. While they are said to be able to recharge an Alkaline battery up to 100 times, their effectiveness is debatable. My take on these devices are that you can sometimes purchase a set of 4 Ni-MH AA batteries and charger for about the same price these devices cost - so why not just buy rechargeable Ni-MH batteries?

Never, never, never, never put a non-rechargeable Alkaline battery into a fast charger. The best chargers will not allow you to start a charge cycle, but dumber fast chargers may. An Alkaline battery is especially not designed to be fast charged, and there is no safety vent. If this battery were allowed to be fast charged, it's internal temperature and pressure could raise to the point of bursting the cell and spewing electrolyte. It's not safe.

 

Charging Do's and Don'ts:
  • Don't recharge non-rechargeable batteries.
  • Don't cross-brand "kit" fast chargers and batteries unless you know for sure it's safe.
  • For maximum capacity, always trickle charge, or top off a fast charge with a trickle charge. Never recharge damaged batteries (electrolyte spewing out, obvious signs of damage, etc).
  • Always discharge a battery completely before recharging it.
  • Always Break-in, or Condition your new batteries if your charger has the capability.
  • Only charge battery chemistries the charger was designed to charge.
  • Never mix rechargeable battery types, sizes, or capacities in a charger, unless it's safe to do so.
  • Always read, understand, and follow the charger manufacturer's instructions, capabilities, and limitations.

 

 


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