The gel cell is similar to the AGM style because the electrolyte is suspended, but different because technically the AGM battery is still considered to be a wet cell. The electrolyte in a GEL cell has a silica additive that causes it to set up or stiffen. The recharge voltages on this type of cell are lower than the other styles of lead acid battery. This is probably the most sensitive cell in terms of adverse reactions to over-voltage charging. Gel Batteries are best used in VERY DEEP cycle application and may last a bit longer in hot weather applications. If the incorrect battery charger is used on a Gel Cell battery poor performance and premature failure is certain.
If you want the low-down on these gel cell batteries, read on. They are very easy to understand when you break them down into individual parts.
Technically they are "Sealed Lead Acid" batteries, but they are made slightly different which makes them a Gel Cell type of battery. The cases are made from a non-conductive material usually consisting of ABS plastic, styrene or polypropylene. A gel battery design is typically a modification of the standard lead acid automotive or marine battery. A gelling agent (often silica) is added to the electrolyte to reduce movement inside the battery case. Gel Cell batteries are considered non-spillable, and can usually be used in any position except upside-down. They usually have a one-way valve acting as a vent for excess gases to escape. A gel cell must be charged at a lower voltage to prevent excess gas from damaging the battery. Fast charging batteries on a conventional automotive charger may permanently damage a Gel Cell Battery. Connection points for power are usually push on spade type of connectors on the lower amperage batteries, or screw and nut type of connection points for higher amperage batteries.
Two 12v 1400mah Nicads
Performance and longevity of rechargeable batteries depends on the quality of the chargers. One type of charger (used only for NiCd) applies a fixed charge rate of about 0.1C (one tenth of the rated capacity). A faster charger takes 3 to 6 hours with a charge rate of about 0.3C.
A charger for NiMH batteries could also accommodate NiCds, but not vice versa because a NiCd charger could overcharge a NiMH battery. Lithium-based chargers require tighter charge algorithms and voltages. Avoid a charge rate over 1C for lithium battery packs because high currents can induce lithium plating. With most lithium packs, a charge above 1C is not possible because the protection circuit limits the amount of current the battery can accept.
Charging of NiCd and NiMH batteries starts with a charge-qualification mode based on battery voltage and temperature. If the battery voltage is less than the charger IC's internal minimum threshold, the IC goes into a charge-pending state. This condition indicates the possibility of a defective or shorted battery pack. To revive a fully depleted pack, the charger IC trickle-charges the battery.
Following qualification, charging continues while monitoring charge time, temperature, and voltage for adherence to the termination criteria. The charging cycle ends with a trickle maintenance-charge that continues until the voltage reaches its end-of charge value.
Precise full charge detection of nickel-based batteries requires ICs that monitor battery voltage and terminate the charge when a certain voltage signature occurs. A drop in voltage signifies that the battery has reached full charge, known as Negative Delta V (NDV).
After full charge, you can trickle charge a NiCd battery to compensate for its self-discharge characteristics. The trickle charge for a NiCd battery ranges between 0.05C and 0.1C. To reduce memory effects there is a trend towards lower trickle charge currents.
This is a more power full battery at 3000 mAh Ni-MH 7.2v
These are a serious battery at 12000 mAh 12amps 1.2v a real punch if you short something.
NiMH battery chargers now use a combination of NDV, voltage plateau, rate-of-temperature-increase (dT/dt), temperature threshold and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge. NiMH batteries that use NDV or the thermal cut-off control tend to deliver higher capacities than those charged by less aggressive methods.
4.8v 600 mAh NI-CD
4.8v 500 mAh NI-CD
When operating loads are too great for alkaline batteries, rechargeable batteries are required. This is the norm for portable devices like notebooks, PDAs, and cell phones. The trick then is to make the rechargeable battery as unobtrusive as possible. The best start for this is to pick cells that complement (or at least do not detract from) the product.
There are two main rechargeable choices, NiMH or Lithium-Ion.
NiMH, which is lower cost than Li-Ion, can make sense when the product's normal use pattern is not unhealthy for the cells. This consideration is particularly important in low-cost products that are unlikely to include sophisticated charging since NiMH cells prefer full charge/discharge cycles. This is suitable for products that are frequently used to exhaustion, such as power tools.
Another pattern that fits NiMH is as alkaline "replacements," where cells are removed from the device when depleted, but then charged in an external charger. This is common in digital cameras, but requires a lot of attention from the consumer.
Alkaline cells are non-rechargeable (notwithstanding claims of late night TV ads) but excel due to their very low self-discharge rate and low cost of implementation (no charger or AC power jack is needed). If power requirements are low, alkaline can be a great choice, but to be used properly, quiescent load, or sleep current, must be reduced with religious conviction.
An often neglected consideration in product design is the interaction between the battery and the system. It is important to match the battery's strengths to the needs of the system. The most common battery types are Alkaline, Nickel Metal Hydride (NiMH), and Lithium-Ion. These are not interchangeable - most products have one "best" choice.
A common mistake is focusing only on the operating efficiency while ignoring "off" or "sleep" current. Even 10s of µA of wasted current can drain cells so that intermittently used products still require frequent cell replacement. Ironically, this design mistake is more common today than it was years ago, since software switches have replaced mechanical switches, which completely disconnected the battery.
A good battery checker is a very useful tool for the workshop.
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