Battery Buying Guide
Deep cycle batteries are ideal for grid hybrid, off-grid and camping and RV applications. Unlike a car battery, they are designed to be discharged to a greater depth and more often. The most common type of batteries for deep cycle applications are Gel, Flooded and Lithium. For RV and camping use, the most common batteries are AGM (Absorbed Glass Mat) and lithium,
There is no overall “best battery”. This is very application specific
Flooded batteries require periodic checking of the fluid level and topping up with distilled water. In deep cycle applications they typically come as 2V cells, packed together into 4, 6 or 8 volt “baskets”.
- Low initial cost
- Tried, tested and proven – we know exactly what to expect
- Are reasonably tolerant to abuse. Can often be recovered (but not always) if chronically undercharged or if they run dry.
- Require monitoring of electrolyte level and topping up. If you are away for long periods of time, they can run dry.
- Under heavy loads, the voltage drops away.
- Relatively low charge rates – typically 10% of the C10 amp hour rating. This means full charging in winter can be a challenge. If charging from a generator, the run times can be quite long.
- Not especially flexible re upgrades. You can only add a second set of the same make and model in parallel. And if the first bank is 3+ years old, the additional bank may be damaged.
- Generally, do not want to discharge below 50% otherwise cycle life is significantly reduced
Absorbed Glass Mat batteries (AGM technology became popular as a sealed lead acid battery for military aircraft and vehicles to reduce weight and improve reliability. The sulfuric acid is absorbed by a very fine fiberglass mat, making the battery spill-proof. AGM (and Gel) are part of the VRLA (valve Regulated Lead Acid) family.
- Faster charging than flooded batteries
- Spill proof
- Maintenance free
- Lighter than other lead acid cells
- Less prone to sulphation
- Stands up to low temperatures
- Good vibration resistance which makes them suitable for RV and camping applications
- Under heavy load do not have the same degree of voltage drop as flooded batteries
- Can discharge down to 80% (not recommended but can)
- Reasonable heat tolerance as the electrolyte cannot evaporate out
- Generally limited to mid-size blocks (30-120 Ah). 2V cells are available but have long lead times as they are not especially popular
- Susceptible to damage if overcharged
- Must be stored in a charged condition
The early gelled lead acid battery developed in the 1950s by Sonnenschein (Germany) became popular in the 1970s. The electrolyte is sulfuric acid with a silica-gelling agent which converts liquid electrolyte into a semi-stiff paste.
- Gel batteries generally last longer than AGM
- Battery can stay in the high performance range during most of its service life before dropping rapidly towards the end of life (AGM, in comparison, fades gradually)
- Good performance at high ambient temperatures
- Less prone to sulfation than flooded batteries
- Needs the correct charge and float voltages
- AGM is superior at low temperatures with better current delivery
- Voltage drop under high load – care with design can mitigate this
- Does release gasses – ventilation required
The two most common lithium chemistries used in deep cycle applications are NMC and LiFePO4. Within that, there are two types:
- Managed Lithium which requires a communications link between the charge source and the battery BMS. This communications link allows the BMS to directly control the charge process. Unfortunately, if the communications link fails, the battery shuts down.
- Unmanaged Lithium which has no communications requirement. The charge source parameters need to be set per manufacturer specifications, but other than that, it looks like a conventional battery. Unmanaged batteries are more likely to be LiFePO4 rather than NMC.
Advantages of Lithium:
- Rapid charge rate – most have the capability of being charged in around 2 hours (obviously you need enough solar to do this)
- Can hold high loads without voltage sag
- Deeper discharge is possible
- High cycle life (generally)
Disadvantages of Lithium:
- Need to be very careful to understand the operating temperature range. Although it may say -10 to +50C, most will start derating up to 15C above and below that. If they get too cold, they won’t charge. To hot and they won’t discharge
NMC or Nickel Manganese Cobalt has high energy density per kilogram and is the battery chemistry of choice for power tools, electric cars, e-bikes and other electric powertrains. NMC has good overall performance and excels on specific energy. This battery is the preferred candidate for the electric vehicle and has the lowest self-heating rate.
Lithium based energy storage systems using NMC include Tesla, Sonenshein and LG
A general disadvantage of NMC is that they require active cooling to prevent thermal runaway. Batteries with basic cooling systems tend to de-rate sharply above 30C
LiFePO4 or Lithium Ferro Phosphate has a lot of advantagesver NMC for stationary energy storage. It has lower energy density (i.e. takes a bit more space), but is far less prone to thermal runaway, can deliver higher current and has a longer life span.
LiFePO4 based energy storage systems include BYD, Gen-Z and PowerPlus
A flow battery is an electrical storage device that is a cross between a conventional battery and a fuel cell. The electrolyte used in most is zinc bromide or sulfuric acid with vanadium salt.
We are not fans of flow batteries due to the lack of a track record in the field. Early adopters have reported numerous failures.
- Novelty factor
- Claimed high cycle life
- Do not deteriorate with age (up to a point)
- Deep discharge
- Electrolyte is very toxic
- They require a regular self-cleaning cycle. This requires a power source and the battery cannot produce power during this cycle. This means an alternate DC power supply is needed (off grid inverters are powered by DC) or two flow batteries.
- High cost compared to alternatives
- Insufficient time in the market to give real confidence