What Solar Panel Size to Charge a 35ah Battery?

A 35ah battery is a good alternative if you only run small appliances and devices. It is light, affordable and still powerful. And compared to larger batteries, you only need smaller solar panels to recharge it whether it is lead acid, lithium or another type.

A 12V 35ah battery can be recharged by two 250 watt solar panels in an hour or by five 100W panels in 5 hours. If the battery is partially discharged at 50%, the charge time will be half that in clear weather.

35ah Battery Solar Panel Requirements

The number of solar panels you need will depend on how depleted the battery is. The performance of the solar panels are also crucial as we shall see.

A 12V 35ah battery is 420 watts, so it takes at least 420 watts solar power to fully recharge it. Because solar panel production fluctuates, you should get a solar array that produces 500 watts.

It does not have to be exactly 500 watts. It can be higher than that. But if your battery needs 420 watts to recharge, a 500W solar array should be enough. Our recommendation is the ACOPOWER 500W Solar Panel Kit because it is an all one solution complete with connectors and a charge controller.

Most solar batteries are 12 volts so that is what we are using. But you may also use a 24V battery if you want more wattage capacity.

• 35ah x 12V = 420W
• 35ah x 24V = 840W

24 volts doubles the watts that a 35ah battery can provide. This is ideal for running appliances and devices for longer periods. But it does mean a longer charge time especially if the battery is completely discharged.

A higher battery voltage also means you have to use a higher solar panel voltage. You cannot charge a 24V battery with a 12V solar panel, but you can use a 24V solar panel to charge a 12V battery. To keep things simple, the PV module voltage must match or be higher than the battery.

How Long Does it Take to Charge a 35ah Battery?

At the top we said that two 250W solar panels can recharge the battery in an hour. However there are several factors that might result in a longer charge time.

A 100W solar panel can recharge a 35ah battery in five hours, but if there are clouds or the sun is low on the horizon, it will take longer. The more depleted the battery the longer it also takes to charge.

All lead acid batteries (FLA, AGM, gel) cannot be used to full capacity. A 35ah battery may be 420 watts, but you can only use 210 watts per charge. That’s because lead acid batteries have to be recharged when capacity drops to 50%. This is known as the depth of discharge (DO). If you use a 120W solar panel, charge time will be a bit faster.

That does not mean you cannot use a lead acid battery below 50%. You can, but it will hurt the battery in the long term. So if you want to use a 35ah lead acid battery, the usable watts is only 50% at a time.

With a lithium battery the discharge rate is 70% to 90%. Some lithium batteries may be discharged up to 100% but even then, it should not be done frequently.

So you can see here how the DOD affects charges. A lead acid 35ah battery may charge faster than lithium because it is only 50% discharged. Two 35ah batteries can also have different recharging times if one is 12V and the other 24V.

Why is My Battery Charging Slowly?

Chances are your battery is fine, but there is not enough current coming in from the solar panels. Without sufficient power from the panels, the battery will not receive charge.

Solar panels regardless of size work the same way. The cells convert the sun’s energy into DC power and it is stored in the battery. Even if you have a good charge controller and all the right cables, the speed of transmission depends entirely on how much sunlight the panels receive. Even if the solar panels and battery are in the right location, it will not charge unless there is enough power coming from the modules.

A 250 watt solar panel can produce 250 watts in ideal weather, but the weather constantly changes so the output will vary. Around noon you may get the maximum watts, but in the morning or late afternoon it might drop to 220 or lower.

What all these means is you cannot make an exact estimate of how long a 35ah battery or any battery will charge. Between the depth of discharge and the weather, a lot of factors affect the charging time.

This does not mean you cannot estimate the duration. You can make some pretty good estimates based on the solar panel output. If that 500W array is close to 500 watts an hour, expect a quick recharge of the depleted battery. If it is cloudy and the solar panels are pulling in 400 watts or less, you will have to wait a little longer.

How Long Will a 35ah Battery Last?

Okay so you have a fully charged 35ah battery. How long can you realistically expect to use it before recharging again? Just like with any other batteries, it depends on the load and how many amps are drawn.

A 35ah battery with a 100% discharge can run a laptop for an hour, or a 42 inch TV for 2 to 3 hours. The battery can run a ceiling fan for 3 hours, a Blu-ray player for 3 to 4 hours or a 12V mini fridge for an hour.

If you only use the battery up to 50% you have to cut the runtime of those electronics in half. While the run time and capacity cannot be compared to larger batteries , it is suitable for short term use, camping, weekend trips etc.

Another way to estimate battery life is to use amps per hour. The amp draw for batteries can be calculated in 20 hour cycles. That is, if the load draws 5 amps per hour, the battery is going to last 20 hours before discharging completely.

This is for a complete discharge. If you follow the 50% recharge rule, the battery has to be recharged in 10 hours. But assuming you opt for a full discharge, the rule of thumb is 5 amps per hour is 20 hours, and 10 amps per hour is around 8 to 9 hours, and so on.

The more amps drawn, the faster the discharge. This is not a battery defect but how they work. No matter the battery type or size, it will always drain faster when you draw more amps. Just like when charging, you cannot 100% accurately predict how long the battery will last.

Which Battery Type is Best For Charging with Solar Panels?

Solar panels can charge lithium, AGM, gel, SLA or FLA batteries without any problem. There is no need to worry about compatibility as long as their voltage is suitable for the panels. Which one you choose depends on your requirements.

A 35ah battery is used to power small loads for short periods, so lead acid is the practical option. Lithium batteries offer better performance, but it costs more. If you are using the battery for only an hour or so per charge, lead acid will be fine.

If you want or need more power you can connect multiple 35ah batteries, but it is better to buy a larger battery, say 50ah or 100ah, than wire several 35ah batteries together. Too many batteries wired together could lead to imbalance as the batteries at the end may not receive the same amount of current that the ones in front do.

Conclusion

To sum it up, a couple of 250W panels is more than enough to recharge a fully depleted 35ah battery. But the other factors mentioned here should also be considered, so keep that in mind if you have this battery and use it often.