Your batteries are at the heart of your boat. The demands on them can be minimal or extreme, but whichever they are an investment and should be looked after accordingly, writes Stuart James.

Several decades ago there was little choice in batteries for boats; they were either commercial or electric vehicle (fork lift or milk float) batteries.

Today, there’s a wide range of choices from lead acid batteries to lithium, although there are variations in all these types.

However, the removal and reinstalling of your batteries is the same process.

Boat battery care: How to remove and reinstall batteries

The installation procedures and the care of your batteries are relatively simple to follow and understand.

Take your time, and please wear protective glasses.

Tools to use:

• Generally, spanners sized between 10mm to 14mm. Some boats might still be in imperial. A socket set might help speed up removal and reinstallation.
• Tools with rubber handles are safer. n A wide-blade screwdriver can help ease the terminals from the post. You might even need to prise open the terminal.
• Hammers with a rubber head. If used for easing the terminal on, it is best to gently tap with the hammer. Excessive force can damage the battery casing or the battery, the battery plates and internal connections.
• A large, thick cloth or towel. Put this over the top of the batteries to ensure a spanner isn’t dropped on the terminals, causing short circuits.
• Gloves are particularly recommended in the removal and installation of your batteries. Make sure you wear them when dealing with flooded batteries.

There’s an order to it

Before you start it’s always a good idea to take photos of the installation and label the leads – that makes it much easier to put things back the right way.

It isn’t always easy to remember the order to disconnect and reconnect the terminals.

When disconnecting the cables from the old battery, disconnect the negative (-ve) first, then the positive (+ve).

When connecting the new battery do it in the reverse order, positive (+ve) followed by negative (-ve).

The same applies to batteries in parallel. Simply think of all the batteries as one battery bank.

Disconnect the negative on one end of the bank, and then the positive on the other end of the bank. Then disconnect all the interconnecting leads.

Take care here, making sure the leads do not touch each other, and make sure you move them safely out of the way, perhaps putting them in a plastic container.

Many marine batteries today have twin terminals; a post and a stud.

Good advice is to use the posts for the connection to the boat’s system, and the studs to interconnect the batteries and any other take-offs.

If batteries have been charging and are warm, the best practice is to exercise patience.

If you are fitting batteries after they have just been charged, take additional care, especially if they are vented, flooded batteries.

Make sure you remove the vents, then blow into the battery to remove the gasses.

Sealed flooded batteries should be allowed to rest and cool down if warm.

Absorbed glass mat (AGM) and gel valve-regulated lead-acid (VRLA) batteries are less dangerous after charging, as the internal gasses should not be omitted, due to the internal valve.

However, you should still exercise additional care.

Flooded batteries should always be kept upright, otherwise there is a risk of electrolyte spillage.

The electrolyte is dilute sulphuric acid, which is highly corrosive.

If it comes into contact with your skin, wash it off immediately. If a spillage occurs in the bilge, apply lots of fresh water to fully dilute it, or bicarbonate of soda to neutralise the acid.

Boat battery care: How tight should the terminals be?

Post (SAE) terminals have a recommended terminal torque of 50-70in/lb (4.2 to 5.8ft/lb).

Without a torque socket, simply tighten the post terminals sufficiently, then turn half a turn more; do not force things.

Examine the battery terminals. If they show signs of serious corrosion, they should be changed as corrosion increases resistance.

Otherwise, simply clean them.

With flooded, vented batteries you should apply a small amount of Vaseline, petroleum jelly or similar.

This protects the terminals from corrosion caused by gases omitting from the battery, or spillage when topping up.

When topping up your batteries, never fill to the top, stop 10mm from the top of the plates.

Many batteries come with terminal covers. Use them where possible.

Keep your battery tops clean and free from any form of dampness, otherwise electrical tracking can occur, and the battery ‘self-discharges’.

Salt water – and there’s usually plenty of it around a boat – is a good conductor of electricity. Keep it well away from batteries.

Charging your batteries

Keeping your batteries in a good, healthy state of charge is vital. Letting your batteries’ state of charge (SOC) fall below 50% really starts to affect the lifespan of the battery (deep cycle batteries can often tolerate this a little bit more).

The boat’s engine will charge your batteries via its alternator.

The alternator settings are normally to a maximum voltage of 14.4V or 28.8V. Lead-acid and lithium batteries have different charge regimes.

Lead Acid boat battery

Initially, the alternator will charge at a high rate until the battery starts to reach its maximum voltage; the flow of the current will then reduce. This normally occurs around 80% SOC.

Putting charge back into severely discharged batteries using just the engine alternator can take a number of hours to get them fully topped up.

Lithium boat battery

Lithium batteries are capable of accepting high charge levels. However, it is possible to exceed the maximum charge level (amps) if your alternator is too powerful for the battery.

Lithium batteries are controlled by a battery management system (BMS). These operate at C1 for 12V and C2 for 24V (the battery C

Rating is the measurement of current in which a battery is charged and discharged).

A general rule of thumb is a 12V 100 lithium battery can accept a 100A charge. Over this, the BMS will switch the battery off for protection.

For a 24V 100 battery, the charge level would be 50A.

So when installing lithium batteries you must check that the output of the alternator will not exceed or overrun the BMS of the battery.

Obviously, if you fit two or more batteries in parallel this halves (or more) the charge going into each battery.

However, there is one more issue with alternator charging of lithium batteries.

The alternator puts out its maximum charge rate at a very low speed. If the engine is on, say tick over, but the alternator is working at a high rate to charge the batteries, overheating of the alternator can occur.

So in many cases, an ‘inline’ DC-DC battery charger should be fitted to reduce the current draw, and thus stop the alternator overcharging.

When deciding on lithium batteries, please do some research first. This also applies to lithium battery manufacturers, but that’s another subject.

Boat battery chargers

Boat battery chargers have advanced significantly over the years as types of batteries have evolved.

Most chargers now have three-stage or even five-stage charges.

The bulk charging stage is the first step, when the maximum current is delivered to the battery.

Absorption is the final stage, which helps the battery’s individual cells balance with one another; this is very important in parallel connections.

Float is when batteries are maintained at the top of their charge. On modern chargers, you can easily select between the different battery technologies, such as flooded, AGM, gel and lithium batteries.

If your charger doesn’t have a lithium setting, select either AGM or gel, as lithium batteries are designed as drop-fit replacements.

AGM will charge to a slightly higher voltage than gel.

A gel setting for lithium batteries is a good idea; the battery might only reach 98% charge but this is fine and will prolong the life of the battery.

Do not use a flooded battery setting for AGM, gel or lithium.

Battery monitoring

Many systems now have boat battery monitoring, either the voltage, or better still the state of charge (SOC) as a percentage of the capacity of the battery.

Battery voltage advises you of the voltage, at that instant, so when connected to the charger (or the engine running), this voltage will be circa 14.0->14.4V when the battery is fully charged.

With no charger or if the engine is not running, the fully charged battery will be 12.6 ->12.8V, but this reduces as load is put on the battery.

With a light load, the voltage needs to be above 12.25V. Lower than 12.00V indicates the battery needs charging.

The battery SOC is the best way to find out when your battery needs charging.

Ideally, try and recharge before 60% SOC for lead acid and 30% SOC for lithium batteries.

A few final points

• Inadequate fuse protection can cause fires on board.
• Make sure the batteries are in a dedicated battery box. This is particularly valid for flooded vented batteries, which also should be vented outside.
• Do not smoke near your batteries. If you can’t see properly use a torch, rather than a cigarette lighter!
• Finally, the ultimate care for your new batteries is to make sure you purchase the most suitable for your application. Flooded leisure batteries are designed for leisure use; AGM and gel batteries are for deep cycle use. Carbon versions of AGM and gel are more suited to boats that are frequently NOT connected to shore power.
• Always check the expected life cycle of your battery before purchasing. The rule of thumb for life cycles are: Leisure 150-200 cycles; AGM 600 cycles; AGM with Carbon 1,000 cycles; Gel 1,000 to 1,500 cycles (with carbon); Lithium 3,000-5,000 cycles depending on the grade and type of cell, but life expectancy also revolves around the life expectancy of the battery management system.

Boat battery care: top tips

• Correct battery installation is important. Inadequate fuse protection can cause fires on board, and batteries should be in a dedicated battery box.
• Use tools with rubber handles. n Always remove the negative lead first and reconnect it last to protect you in case a tool touches an earthed component during terminal removal.
• To ensure sufficient charging, the tops of the batteries should be kept clean. Any vented acid could combine with salt water moisture and produce an electrically conductive solution which could lead to electricity discharging across the top of a battery. Salt water is corrosive, so do not allow it near connections or wiring.
• Do not overtighten terminal posts or you may distort them. Never hammer battery connectors into place – you could damage the bus bar supporting the internal plates.
• Keep battery terminals free from corrosion as any calcium deposits will act as a resistor, preventing your batteries from fully charging.
• Clean terminals with wire wool or emery paper. A thin layer of petroleum jelly or terminal protectors can help prevent corrosion.
• Wear protective gloves and goggles when handling batteries. Electrolyte is a mix of sulphuric acid and water, which you don’t want on your skin.
• Ensure batteries are kept upright so electrolyte can’t leak. If you do spill any, use bicarbonate of soda to neutralise it.
• Batteries can be a fire risk; keep metal objects and sparks away from batteries and do not lay objects on top of batteries.

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