What makes some batteries rechargeable, and what do those letters describing them mean?
Historically, the word cell was used to describe one electrochemical unit comprising a positive electrode (cathode), electrolytes, and a negative electrode (anode). The redox reaction taking place in the electrolyte created a difference in electrical potential, translating to voltage, between the anode and cathode. A battery was once a group of cells wired in series, but today, 'battery' is used to encompass individual cells as well.
Why are only some batteries rechargeable?
Disposable batteries, also known as primary batteries, are generally cheaper to manufacture and rely on chemical reactions that can’t be reversed. Secondary batteries utilise metals and electrolytes that can be restored to their original state by connecting them to an electricity supply of a higher voltage.
Although the chemical processes used by batteries are different, one analogy is that you can't unstrike a match, but you can crystallise salt out of a saline solution, and then dissolve it again.
Here are five common chemistries of rechargeable batteries.
NiCd (nickel/cadmium)
Relatively cheap and easy to produce, these batteries discharge relatively quickly in storage and are only used in limited applications today.
NiMH (nickel/metal hydride)
A step up from NiCd, this chemistry includes a hydrogen-absorbing intermetallic alloy instead of cadmium in its anode. This gives it a far better shelf-life, meaning batteries can be sold pre-charged.
Li-ion (lithium ion)
This is a very popular variety for everything from power tools to mobile phone batteries. They hold charge extremely well during storage, and they don't suffer from memory effect.
LiFePO4 (lithium iron phosphate)
This is one of the latest developments in rechargeable technology. Deep-cycle LiFePO batteries include sophisticated onboard electronics to manage charge and provide overload protection plus other safety features.
SLA (sealed lead-acid)
Using a similar chemistry to a typical car battery, which uses lead plates and sulphuric acid, the sealed version contains acid that usually takes the form of a thick paste. Unlike vented car batteries, any gases produced in normal use are held inside the battery casing. The lead component makes them very heavy, but deep-cycle variants are less expensive than LiFePO4 equivalents.
See below to learn about intelligent charging.
Loads of batteries, loads of chargers. How do you choose what you need?
Regular Discharge, or Emergency Backup
The primary factor in choosing a battery charger is, what do you want to use it for. If your batteries are used in an emergency situation only (eg, something that runs only during a blackout), versus regular use (eg, powering your off-grid system at night) then your charging requirements are vastly different. If you're regularly discharging your battery you'll need to make sure the charger has enough power to recharge it in the interval you have not using the battery (ie, if you use it every night, you must be able to recharge it within 8-10 hours for instance). If your battery is used purely in emergency situations (such as for an emergency lighting system), you can often proceed with a trickle charger. It may take a long time to recharge your battery, but you generally only need to use it in uncommon situations, so the recharge time is of little significance.
"Trickle" Battery Charging
Most batteries naturally discharge over time. You can't leave them on a shelf for extended periods, and expect them to operate as designed when you decide to finally use them. This natural discharge is overcome by the practice of trickle charging. A trickle charger basically counteracts the natural discharge effects, and ensures your batteries are ready to use all the time. If you don't keep them topped up, they'll slowly become less and less useful, until ultimately they won't work any longer (and often can't be rescued from this state). Trickle chargers aren't usually powerful enough to recharge a drained battery quickly, and may take 24hrs or more to recharge it fully. For an emergency power source, this is fine. You keep it charged, use it in an emergency, and when the power is available again, it can recharge fully, ready for next time.
Standard Battery Charging
A standard battery charger takes a moderate approach, balancing charge rate with charge time, and heat. It will usually recharge a battery overnight (or during the course of a day if it's used overnight). They'll provide a solid charge.
Fast Battery Charging
Fast charging is actually a relative term, based on the size of the battery (or batteries) you're charging. If a charger has the ability to charge from almost flat within say, a few hours, that would be considered a fast charger. You can usually discharge your batteries at a FAR greater rate than you can recharge them, so being able to recharge as quickly as possible is essential in various situations. There are limits to how fast a battery can be safely recharged, as excessive charging rates will create too much heat, which can destroy the battery.
Multi-State Battery Chargers
You don't always have to determine what sort of charging you're doing in advance. A multi-state charger will allow you to set a charging rate suitable for what you're doing at the time. Whether it's fast charging so you can get your car started, trickle charging for your boat battery while it's in the garage for the winter, or somewhere in the middle. Multi-state chargers allow you to select the most appropriate rate for what you're doing at the time.
Intelligent Multi-State Battery Chargers
An intelligent battery charger is a highly clever, adaptive style charger. These chargers can do the thinking for you, allowing a "set and forget" approach to battery charging. They monitor the battery voltage and adjust the charging current accordingly. Some will even have an optional temperature sensor, to keep an eye on battery temperature to make sure the heat generated doesn't become an issue. You can usually override the automatic selection of charge mode if you do want to take control, but you can usually just connect the terminals and let it figure things out for itself. The charger will then charge as fast as practical to do so, then slow things down eventually to a trickle, to maintain the battery's charge for you. Intelligent chargers are definitely the best option if you're unsure what charge condition your battery is in, or you don't really understand electrical principles - they're safe and effective for just about anyone to use.
If you're unsure which battery charger to choose, our friendly team can help you make the right choice. Just consider what sort of batteries you're charging, as that's probably the first question our team will ask.