Frequently Asked Questions - Battery Charging

How do I size a battery charger for my battery(ies)?

We at deal with lead acid batteries in all their forms: flooded, Absorbed Glass Mat (AGM), and Gel. Most battery manufacturers recommend about 25% of battery capacity (C) for an ideal charger output. So, a 100 amp hour battery would call for about 100 x .25, or 25 amps. This rate would charge the battery relatively quickly, while not gassing excessively (wet batteries), or overheating the batteries. A lower amp charger would work fine, just take a little longer. A higher amp charger will charge the batteries quicker, but may shorten their life somewhat, if much above the 50% level (a 50 amp charger in this case). The microprocessor controlled charges mitigate the higher amp charge rate somewhat, but care should still be taken. Some batteries are marked with a charge rate for slow charging, and fast charging, which are different from the above recommendation, but our level is generally safe in the lead acid battery types. Some high performance and high quality AGM batteries like the Hawker or Stinger and some of the Odysseys can be charged at up to C (capacity) without damage. For example, a 72 amp hour Stinger SP1700 cab be charged at 70 plus amps, and Stinger even lists a 100 amp charge time, but these charge rates are the exception, not the rule.

Floating batteries (long term maintenance on the 'smart' chargers) can use smaller capacity chargers, and many are designed just for this purpose. A 750 milliamp (.75 amp) to 2 amp microprocessor charger can float motorcycle/ATV/jetski batteries, or automotive size batteries. They are a little small to bulk charge an automotive battery, as this would take some time. I know, long answer for a short question.

Can I use the same charger on all lead acid batteries?

AMG and flooded (wet) batteries can generally use the same charge profile, hence the frequent change in RV and marine applications to the sealed lead acid AGM types to eliminate the maintenance issues, without requiring a charger change. Some chargers differentiate between AGM and flooded with a switch, but most battery manufacturers have no problem with a flooded cell type charger or alternator as the charging source.

The problem arises with a true Gel battery. Some people use the term 'Gel' generically to refer to all sealed lead acid batteries, probably because these were the first types. This is not correct, and causes confusion. A true gel battery requires a different charge profile involving lower voltages for the absorption and float stages. Some chargers are selectable, and some are gel safe, but this must be determined to avoid battery damage. Using the higher voltage of a flooded or AGM charger causes bubbles in the gelled electrolyte, which do not diminish after charging, and these bubbles reduce electrolyte contact with the lead plates which reduces battery capacity or destroys it completely.

What's the difference between a discount store battery charger and an industrially rated unit?

The discount store type battery charger, while possibly (probably) being microprocessor controlled, uses lower quality components and cooling methods, as they are designed with low cost and occasional use as their primary design criteria. The industrial units have higher quality components and heat management in the design, and most (all of the chargers we carry) are rated with a 100% duty cycle, which means they can run at full amp capacity for extended periods. They tend to be more expensive, but generally not excessive. Some of the discount store type chargers specifically say not to use them in a daily use or industrial environment in their owners manuals. We will not carry a battery charger that says something like that.

Can I estimate recharge time for a given battery and charger?

You can estimate recharge time by dividing the amps to be replaced in the battery by the charger amp output. For example, you have a 100 amp hour deep cycle marine battery that is 50% down, therefore giving you 50 amps to replace. If you have a 10 amp charger, you divide the 50 amps by the 10 amp charge rate of the charger, and get 5 hours. In reality, the multistage chargers taper the current after about 80% to 85% of the bulk charging is done, so it takes a little longer (closer to 6 hours in this case).

How do the microprocessor chargers differ from 'trickle' chargers?

Trickle chargers are generally considered to be the older, fixed amp output (though low output) non-intelligent chargers. They do not take any information from the battery, and simply put out a fixed rate. This can be enough to overcharge over time, and had to be monitored to be safe for the battery. The microprocessor controlled chargers actually take information from the battery, and regulate the output down to milliamps, to maintain a set floating voltage, which compensates for internal battery drain, to maintain a fully charged state without overcharging. Lead acid batteries can be held in this state for long periods (indefinitely), though the flooded type still needs to have the electrolyte checked occasionally, due to evaporation.

For more information, see our Tutorials on charging and batteries