Can You Charge a Battery From Two Sources?

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Waiting for a solar battery to recharge is like watching paint dry, it takes too long. So the question, can you charge a battery from two sources? Maybe even more? In this guide we will explain what will happen if you use multiple chargers and if it is worth it.

Charging a battery from multiple sources is possible but the benefits are limited. During the bulk phase charging will be faster, but when it reaches 80%, only limited amounts of current will be accepted, slowing the charge down. Some batteries are also designed to charge only at certain rates.

Will Multiple Sources Speed Up Battery Charging?

The answer to this question is yes and no. Yes in that you might see faster charging early, but later on it will slow down as if you are using just one charger.

An empty 100ah battery might take 8 hours to reach the bulk phase with a 10 amp charger. Having another charger lets you reach this stage faster. But once you get to the absorption phase, the voltage level is held at its peak and slows.

To understand how battery charging works, we have to explore the different stages of battery charging. The following is a brief explanation of each stage, but there is more information here.

Battery Charging Stages Explained

There are three basic stages, the bulk, absorption and the float. These apply to all batteries used in solar systems.

Stage 1 Bulk. The bulk stage makes up 80% of the charge. The battery voltage goes up and the current level remains constant. During this phase the charge will supply as much current as the battery can handle.

The bulk stage is where two chargers can help. The battery can accept a lot of current and having multiple chargers is going to make a difference.

Stage 2 Absorption. The absorption phase occurs during the last 20% of charging. The voltage is fixed at the charger set point, usually 14.1 to 14.8 VDC. The current is decreased during this period, so having two chargers is not going to speed up the process. This is also known as the equalization stage.

Stage 3 Float. The float stage has the voltage set at 13.0 to 13.8 VDC. The current is dropped to less than a percent of the battery capacity. The float phase is used to keep a battery fully charged.

Why Two Sources Does Not Always Mean Faster Charging

The explanations given above shows you why having multiple chargers is not always effective. It has nothing to do with the device efficiency, but how a battery operates.

Simply put, when a battery reaches the absorption stage in charging, voltage and current are restricted. You can have as many chargers as you want and it will not make a dent because the battery will not accept anymore power.

In flooded lead acid batteries, the absorption phase means the current tapers off. The voltage is also fixed. The absorption stage is in the final stages of the charge, and it can take 3 to 4 hours to complete in an empty battery.

How Long Does It Take to Charge a Battery?

There are a lot of factors that determine how long this will take. If you use solar panels to charge a battery , it will depend on the sun hours and the panel capacity. But if you have a battery charger, the calculations are different.

To find out the battery recharge time, divide the amps hours that need to be recharged by 90% of the charger output. It looks like this:

Amps hours needed / 90% charger amp output = approximate charging time

A 100ah battery has been discharged at 30%, meaning it needs 30 amps to be fully recharged. If you have a 10 amp charger, divide 10 amps by 90% of 10, which is 9 (.9 x 10= 9).

10 / 9 = 1.1

It will take 1.1 hours to add 30 amps to the battery and fully recharge it.

Note that the more deeply discharged a battery is, the longer it will take to fill. Even if you have two sources, the charge time will take longer because that is how all batteries work.

Why Batteries Take So Long to Recharge

Basically the more amps drawn from the battery, the faster it loses power. And the deeper a battery is discharged the longer it will take to recharge. Lithium batteries can be fully discharged to 100% which is great if you need the power. But the charging time will take longer.

Deep cycle lead acid batteries usually have a discharge rate of 50%, but AGM and gel may be discharged at a higher percentage. Whatever the rate is, the formula given above will not be as accurate because the batteries will need more time.

Age is another factor that affects charge time. Batteries get worn out with prolonged use. Even if you have several chargers, an old battery can only handle a limited amount of current and voltage.

New batteries last longer and recharge faster. Through wear and tear, the opposite happens and the charge runs out faster and recharging takes more time.

Is There Any Risk in Using Several Battery Chargers?

This is a common question that comes up that needs to be addressed. Will the system experience conflicts if you connect two battery chargers?

Two chargers should not cause conflict provided both are compatible with the battery. But if one charger has higher specs than the other, it might overpower the smaller charger.

An example of this would be adding a portable solar charger to an RV solar system. If your camper has a solar array, it will probably overpower the portable charger rather than work with it to make charging faster.

In some cases there might not be any issues, but there will be little to no speed improvement. The results will vary from battery to battery, but for large solar systems, adding a solar charger has a negligible effect.

Some batteries however, do suggest limits to the charging speed. A rule of thumb is that batteries should be charged at 10% of its amp rating. A 100ah battery for instance, should be charged at 10 amps, not more.

Can you use a higher amp charger as in 15 to 25 amps? Yes it is possible. But it might cause long term ill effects like shortening the battery life cycle. It is always better to go with the recommended amp charge rate.

How to Speed Up Battery Charging

Using several chargers can help but only so far. If you really want to make it faster, you have to charge and recharge batteries the right way.

Discharge at the right rate. For FLA that is usually 50% maximum. When the capacity drops to 50% it is time to recharge. Of course it is not always practical or possible to follow this guideline, but do your best to do so. It will prolong the battery lifespan.

Know when to top off the battery. Topping off does not always mean charging up to 100%. In fact some batteries are better off if you stop at 85%. Ideally you should recharge at 50% and top if off at 85% for deep cycle batteries.

Do not overwork the batteries. Do not run the load to its limit. If the capacity is 100ah the maximum load load should be 85ah, 90ah tops. And you should only be doing this with a lithium battery anyway.

Maintenance and upkeep. Lithium batteries do not require maintenance, but lead acid batteries do. Knowing how to clean and manage the batteries is good for the battery and might make charging go faster.

Use a quality charger. This one is obvious but bears repeating. A good charger with high amps (acceptable to the battery of course) is essential. If you really want to use two chargers, make sure they are compatible with the battery and have similar specs.

Conclusion

Waiting for batteries to recharge is a pain, but it is something all solar power users have to deal with. Adding another charge source might help, but really all it will do is speed up the bulk phase, only to slow down later. The good news is batteries are getting more efficient, and charge time will become better too.