One of the most common questions we get is "how long will my batteries last" or "how long can I free camp for". These questions are some of the most challenging things to work out. There are many contributing factors to this with loads of variables, so we'll only go over the main ones.
Limitations of batteries.
When using AGM batteries, it's recommended to only discharge to 50%. This is usually around 12·2-12·3 volts.
When using 100ah AGM batteries, this typically means that you can use around 50ah of the available 100ah before you begin to damage and shorten the battery's life.
Batteries degrade over time, meaning a 12-month-old 100ah battery does not put out as much sustained power as a 100ah 12-day-old battery. Irrespective of maintenance or use, for lack of a better phrase, batteries "go off", like a carton of milk.
Any time spent below 50% is also shortening the life of your AGM battery. Most battery suppliers will say their batteries last around 200-300 cycles if you're looking after the AGM properly. This means not subjecting it to severe vibrations, temperature extremes or overly high draws such as running large inverters off small battery banks.
BMPRO’s Battery Management Systems, such as the BatteryPlus 35 and Genius II ranges, have a low voltage disconnect, or LVD. This stops the main system from drawing power below 10·5 volts on the battery. This feature is there to help protect against immediate short term damage to the batteries and shouldn’t be used as the determining factor for the batteries being flat. Attempting to recharge severely depleted batteries can lead to battery failure. Essentially, after resting the battery from a charge, 12·8 volts is fully charged, 12·24 volts is 50% with 11·8 volts being 0%. Anything wired directly to the batteries will not shut down though and can continue drawing power from the system.
Failure to look after a battery, subjecting it to heavy use or harsh conditions, dropping it below 50% regularly or leaving it below 50% for long periods will shorten the life drastically and often lead to battery failure within a few months.
When using 100ah lithium batteries, you can typically use around 80-90%. Some battery manufacturers limit the battery's output so that you only use a fraction of what they rate the battery as, while others give you a rating that shows what you can use. This means that some 120ah batteries may only have 100 or 105ah usable on them while others can use all 120ah. Refer to our article "Which Lithium batteries should I get?".
When looking at the usable amount for lithium, it varies so wildly between manufacturers that it's not possible to give an accurate figure. It's best to look at the voltage that the manufacturer says the internal Battery Management System shuts down. Keep in mind that dropping a battery to this level may require a jump start or pulse mode to bring the battery out of sleep mode. This is an emergency cutoff and should not be used as an indicator of battery level.
When connected to a BP35 set to lithium, main output loads disconnect at 12v in an attempt to prevent the lithium batteries from going to protection mode.
Lithium batteries do not hold "more power", as often advertised. Instead, you can use more of the total power comfortably. With 2 x 100ah AGM batteries, you have 100-120ah you can use before reaching your 40-50% cutoff. With lithium, you usually see around 180ah. However, if you use both the AGM and the lithium until they stop supplying and the power shuts down, you can only pull a maximum of 200ah from each, depending on how the manufacturer of that battery chooses to rate their battery.
There are several items that have standby draws that often appear insignificant to begin with. Still, as these draws are often running 24/7, they add up very quickly. Items such as heater displays, satellite trackers, RV WiFi, inverters, antenna amplifiers and satellite controllers & decoders are the most common culprits. They can have standby currents as little as 200 milliamps (200ma is 0·2 amps) and as much as 1 amp. Something like a TV amplifier uses up to 0·3 amps per hour when switched on, whether you are watching TV or not. A draw as small as this will still end up using around 8ah per day. That’s nearly 10% of your total capacity, 16%of your recommended usable capacity, gone in one small standby current.
The easiest way to manage these draws is to ensure that they are switched off. TV Amplifiers will always have a button. Satellite systems can always be unplugged, as can RV WiFi units. Inverters should always be switched off unless they are needed for 230v power inside the van. Taking these steps will ensure that you are minimising the power wasted in your van and gives your solar system its best chance of keeping up with the needs of the van.
Now that the other main factors have been taken into account, you can look at the amount of power you are actually intending to use.
Everything in the van is using 12v power and draining the battery when you are not connected to mains power. The majority of the time, you are only slowing the decline to flat when running without mains power. It’s a good idea before you head off to take the time to look at what lights and appliances use the most power. It’ll make it far easier to manage your power usage when you have a better idea of what things will use.
There are some things like strip lights and bulkhead lighting which look great aesthetically, but often can be too high of a draw when relying on the battery. Remember that a single LED is very energy efficient, however, a row of 100 of them is not. External lights are another fairly high draw item, as are fan hatches with heating elements in them.
Compressor fridges use a lot of power over the course of 24 hours, more so in higher temperatures, when the door is being opened frequently or when large quantities of items are being added at once and need to be cooled. Some 3-way fridges have very high draws when running on gas though too, so be aware of these. Some newer models have things like performance mode where there are multiple fans and heating elements in the door seals which draw a lot of power when left on. On mains, this won’t be an issue, but when relying on battery power this can be more of a drain than your batteries can keep up with for long. These optional items should be switched off if not required to save power whenever possible.
It’s not uncommon for a medium-sized van with a compressor fridge and most lights on, to be able to use between 12 and 15 amps per hour without TV’s or other high draw items. Your batteries won’t last long at this rate. To be precise, most 100ah AGM batteries would be depleted after 5 or 6 hours at that rate of draw.
Learn how much power everything uses, then watch your loads and time to empty if they are available on your displays to get an accurate idea of how much, or little, power you have left.
More important than managing the amount of power you are using is to take into account the amount of power you can get back each day. This varies greatly with temperature, weather, panel cleanliness and location.
Many people will try to rely on solar without taking into account that they are using all of the power they are creating during the day and not storing the power for the evening. In most vans, you will need to generate around 3 times what you are using per hour in order to break even over the course of 24 hours.
Generators are a great addition to your kit if you can afford to carry one. Selecting the correctly sized unit will allow you to run everything in your van, charge your battery and run items such as your Air Conditioner at the same time.
Refer to our articles on solar for more specific information and tips.