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If a 50ah battery is too small and 100ah is too heavy, 75ah is a good compromise. You have plenty of power without taking too much space in your camper or boat. But how many solar panels will you need to recharge a 75ah battery? As we will explain you have many options.
A 200 watt solar panel can charge a 75h battery with 5 hours of sunlight. However it might be a good idea to use a 250 watt solar panel if the weather is overcast or if the battery needs to be charged under 5 hours.
How Many Solar Panels Do I Need to Charge a 75ah Battery?
There are two questions that need to be answered, how many solar panels and what voltage are needed. Both will depend on what type of 75ah battery you have.
A 12V 75ah battery can be recharged by a 12V or 24V 200W solar panel. A 24V 75ah battery should be paired with a 24V 200W solar panel, because the panel voltage has to be greater than that of the battery
You can use a higher voltage solar panel to charge a low voltage battery, but you should have an MPPT charge controller. If you use a PWM controller, it will pull the panel voltage down so it is similar to the battery, wasting its power. With an MPPT, the solar panel runs at the highest voltage.
A quality 12V 75ah battery such as the Mighty Max holds 900 watts so one 200W panel is enough.
200 watts an hour x 5 sunlight hours = 1000 watts
You can also use 2 x 100W solar panels or any array that produces at least 1000 watts. If the battery is not fully discharged you do not even need 1000 watts. Lead acid batteries should not be discharged below 50%, so only 500 watts are needed.
If you buy a 75ah battery, plan its usage and recharging. If you are taking the battery in your camper, how many solar panels can you install? Will you use the solar panels on your RV roof or get one specifically for the battery?
In other words you have a lot of options for charging the battery with solar panels. And this has a direct effect on the charge time.
How Long Does it Take to Charge a 75ah Battery?
The charge time depends on three factors: how empty the battery is, the solar panel size and the weather. A fully discharged 75ah battery can be recharged in 5 hours by a 200 watt solar panel with 5 hours of sunlight. A 300 watt solar panel will only take 4 hours. The higher the panel watt rating, the faster the battery charge.=
The first thing you have to look at is the battery capacity, specifically how much power is left. As we mentioned, you should never fully discharge a lead acid battery. As you will see this has a significant effect on how much power you can load and how long it will take to top the battery.
How Depleted is the Battery?
In the examples above the assumption is the battery is completely discharged. With lithium batteries you can do that, but with lead acid you are not supposed to let the capacity drop below 50%. Other batteries like AGM have a 70% discharge rate so this affects the charge time.
If you have a fully discharged 12V 75ah battery and a 300 watt solar panel, charging time could be 4 and half hours. This assumes the solar panel produces 300 watts an hour.
Weather and other factors affect solar panel output, so it might be 280 watts or less depending on the weather and season. Even so the battery should be recharged in less than five hours.
If the battery is partly discharged at 50%, the charge time will be different.
75ah is 900 watts but with a 50% DOD only 450 watts is required. A 200 watt solar panel can recharge it in 3 hours and a 300 watt panel in an hour and half.
Here you can see the pros and cons of using lithium and lead acid batteries. You can use a lithium battery fully but it will take longer to charge. With lead acid you can only use half the capacity, but the recharging time is faster.
The Weather and Battery Charge Time
Whether you are using solar panels to power a trolling battery or RV, the weather will always be a factor. On average you will get 5 hours of sun but during summer you could get 6 to 7 hours, and in winter you will get very little sunshine.
Unless you go camping during winter, you should get 5 hours on average. But do check the local weather if there is the possibility of a thunderstorm. If you have to recharge the battery as fast as possible, use the largest solar panel you can carry.
Once you have the charge time figured out, you also have to get an idea of how long a 75ah battery will last. And the answer depends on a lot of factors.
How Long Will a 75ah Battery Last?
The runtime depends on how the battery is used. The more amps drawn the faster the battery gets discharged, so you need to keep this mind.
A 75ah battery can theoretically run a 450 watt load for two hours before it is fully charged. In reality it will probably be less than two hours. If you lower the amps drawn per hour, the closer the battery will be to its theoretical runtime.
Battery Watts and Amps Runtime Calculation. There are two ways to determine how long the battery will last, watt load or amps per hour drawn. A 12V 75ah has 900 watts while a 24V battery has 1800 watts. By adding the total watt load per hour you can estimate how long the battery will last.
Appliance power consumption can also be computed in amps drawn. If your total draw is 10 amps an hour, the battery should last for approximately 7 hours. If the load is 20 amps the battery will be depleted in 3 and half hours.
Again these are estimates only, and you also need to factor the inverter and its efficiency. An inverter is needed to turn solar DC energy into AC which appliances use. But during the conversion process some energy is lost.
If your 75ah battery is 24 volts, its watt capacity is doubled and the amps drawn is reduced by 50%. So the battery will run longer while running a heavier load. 24V batteries cost more than 12V, but they do provide extra capacity so you have to decide if it is worth the price.
What Can I Run with a 75ah Battery?
A 12V 75ah battery can run a 900 watt load or a 75 amp draw for an hour. A 24V 75ah battery has a 1800 watt capacity and lower amp draw. If it is a lead acid battery the available watts is 450 per charge. If this is what you prefer, there is the Mighty Max 100ah Gel Battery available in 24V.
You can connect any power load to the battery as long as it does not exceed the battery. While you can theoretically load as much as 900 watts, you should limit it to 850 watts so there is reserve power in case of a power surge. A sudden power spike is another reason why you should ground your solar panels.
Even if you have a 50% DOD deep cycle battery, a 450 watt capacity should be enough for a day of camping. You can run a small fan, recharge your mobile phone and run a laptop for an hour. For a 3 day camping trip you will need more than a 75ah battery especially if you are going to use several appliances.
You can use two 75ah batteries and if they are lithium you have 3600 watts. Pair this with a 200 watt solar panel and you can recharge the battery in a day. You can use the solar panel to run your appliances and let the battery take over later.
Of course this assumes the solar panels are sufficient to run your power load. While 200 watts is enough to charge a 75ah battery, you will need more if you have two 75ah batteries and more than a thousand watt load.
While 100ah batteries are the most popular size, 75ah batteries are more than serviceable and versatile. Whether you go with lithium or lead acid is up to you, and remember that the battery type affects the charging time.