It was not that long ago that going camping or going off-grid meant throwing a tent in the car, some food, a stove, an esky, and after two days, once the ice melted, you drank warm beer.
Today, to get away, people want (not need) Starlink for internet, a 12-volt fridge to keep the food and drinks cold, and an induction cooktop to cook without the wind blowing the flame out. On top of that, they want their phones charged and to light up the campsite like a football stadium, so it is safe to move around.
All this requires a lot of power, leading people to install lithium batteries as part of their setup. Soon, one battery is not enough, and adding a second lithium battery becomes a new quest.
Before I discuss how to add a second lithium battery, let’s address one of the biggest myths surrounding the topic. It has often been said that you must add the same brand and capacity at the same time. This is false for lithium batteries. It is true for older lead acid technologies, but not lithium. To dispel this myth, watch Will Prowse’s YouTube video titled “Current Sharing 101”.
I recently went through this exact quest of adding a second battery to my caravan, so let me save you some time. My caravan had a 200Ah lithium phosphate battery, which was managing fine as long as the sun shone and my solar panels recharged it. I could not run my air conditioning overnight (a first-world problem), and if it was overcast, I was in trouble when off-grid. Eventually, I settled on a Bluetti B1232 314Ah additional battery.
What size second lithium battery should you get?
The shortest answer to this question is: the biggest battery you can afford that will fit in the space you have. As more gadgets and wants become available — like Starlink and bigger TVs — you need more and more power while travelling.
My advice is to get more than you currently need. Do keep in mind that the solar or vehicle charging of your battery bank needs to be able to replenish your capacity in around eight hours. So, unless you plan to recharge from the grid regularly, don’t buy more than you can recharge, as it will be a waste of money. Lithium batteries are much lighter than lead acid, but still consider how much weight you are adding.
For my setup, I could fit a 300Ah (amp-hour) battery. Some of the appliances you can run with such a battery include:
- Air conditioner
- Fridge
- Microwave oven
- TV
- Kettle
- LED light
- Laptop
Which brand and model lithium battery should you buy?
Batteries with the same capacity can vary widely in price depending on their features. Features to consider are:
- Bluetooth: Can you control and monitor the battery via a Bluetooth connection? Having had battery power issues on the road, I would not buy a battery without Bluetooth. You can see charging/discharging state, battery recharge cycles, cell faults, voltage, temperature, etc.
- Battery cycles indicate how many times a battery can be recharged before the battery capacity drops below 80% of its original capacity. The bigger the number, the more years it will last.
- Battery Management System or BMS protection keeps you safe by monitoring overcharge & over-discharge protection, short-circuit & thermal shutdown, temperature-controlled charging and high/low voltage cutoff.
- Max charge current refers to how quickly you can charge the battery. Note that the biggest affordable/practical chargers are only around 100A.
- Max discharge current refers to how much power you can draw from the battery at once.
- Weatherproofing is good if your battery is used on a boat or may be exposed to rain.
- In very cold conditions, an inbuilt heater will keep a Lithium battery working. Useful in snow regions.
- A reset button allows the battery to be turned on/off and reset. Useful if you drain your lithium battery and want to get it going again easily.
- Batteries of the same capacity can have different weights, with lighter ones being better for mobile setups.
- Warranty for peace of mind.
In the table below are some of the highest-selling, cheapest, and highest-quality lithium phosphate batteries.
| Kings 300Ah | Bluetti B1232 314Ah | Enerdrive B-Tec 300Ah | |
|---|---|---|---|
| Price | $899 | $1,600 | $3,999 |
| Bluetooth | No | Yes | Yes |
| Cycles | 2,000 | 6,000 | 6,000 |
| Max charge | 300 | 140 | 300 |
| Max discharge | 300 | 140 | 300 |
| Weatherproof | No | IP65 | IP67 |
| Heater | No | Yes | Yes |
| Reset Button | No | Yes | Yes |
| Weight kg | 27 | 29 | 37 |
| Warranty years | 3 | 5 | 5 |
| Mounting brackets | No | Yes | Yes |
The Bluetti holds its own in this comparison, except for maximum discharge. It can peak to 310Ah for eight seconds, 140Ah continuous, but that does not help if running a 2600W inverter, which requires a minimum of 204Ah continuous output. I would not recommend a single Bluetti for high outputs.
However, if you are running two 300Ah batteries, the load is split equally between each, so the draw on each battery is 102Ah, well within its limits. In my situation, with a 200Ah and 300Ah battery, the Bluetti will draw 122Ah (equation: 300/(200+300).
The Bluetti positives for my needs were warranty, Bluetooth, weight and cycles. On the Bluetti app, you can see battery charge percentage, real-time voltage and current, battery temperature, remaining charge/discharge time, and battery status.
Does cable thickness matter in connecting a lithium battery?
Adding an extra battery requires two extra battery cables.
The lower your battery voltage, the higher the amps required through a cable to meet a certain wattage requirement. If we use our example above, 2,600 watts would require 204 amps (2600 watts divided by 12.8 volts) to run through a cable. This means that unless you have a very thick cable, the cable will overheat and hopefully blow a fuse first before melting or bursting into flames. The cable’s length also matters. A longer cable creates more resistance, so the longer the cable, the thicker it needs to be. Enerdrive has a great guide on cable sizes.
Cable size can be measured in three different ways: AWG (American Wire Gauge), diameter or cross-section. In Australian retailers, you will normally find the AWG rating being used. For my application, I used the King’s 1 AWG battery linking 350mm cables, which have a diameter of 8.25mm.
How do I connect a second Lithium battery?
Adding two 12-volt batteries together can yield two outcomes, depending on how you connect the positive and negative terminals.
If you connect the batteries in series, connecting the positive of one to the negative of the other, you will get 24 volts. In my situation of adding a second battery to a 12-volt setup, I must connect the batteries in parallel so the voltage remains at 12 volts. This is positive-to-positive and negative-to-negative between batteries. You will need two cables, the same length, long enough to reach each terminal.
Before you connect your new second battery, and yes, you can mix an older battery with a new battery, you must fully charge each battery. You must then, independent of your 12-volt setup, connect the two batteries in parallel with no other connections. Positive to positive, negative to negative. This process, which you leave for 12 to 24 hours, balances the voltage in all the battery cells. This same process should be followed when setting up a brand-new 12-volt system.
Once your batteries are balanced, you are ready to reconnect them to your setup. Best practice is to connect the positive lead from your setup to one battery and the negative lead to the other battery. Your two parallel cables stay connected, positive to positive and negative to negative. If you have a 12-volt master switch, turn it back on, and you are done.
It is legal for you to wire your own 12-volt systems (unlike 240-volt). However, if you are not confident, please contact an auto electrician to do the work for you. Be careful not to short-circuit the batteries by touching the positive and negative wires together.
In my case, adding the second battery was pretty simple, and now I can look forward to the extra security of having more power off-grid when the sun does not shine, and who knows, on a hot night, I might even run the air conditioning.
The post How to add a second lithium battery to your off-grid system appeared first on GadgetGuy.
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