FAQs

Dual Battery Options in Boats or Motorhomes

Many industries have similarities and principals of one that can carry over to the other. Marine industry or marine electricians to be precise can carry over their ideologies to motorhomes and remote off-grid setups.

Boats have utilised batteries since their invention to power electronic equipment onboard including starting an engine. Later people added living luxuries like lights, heaters, pumps, cooking. More recently GPS, Radar, Satellite Radio, auto-pilot, air condition, these are high demand and long duration discharge applications. Batteries have been used in full electric and hybrid electric installations also. These types of batteries all have a different purpose and you'd typically use one battery type for each installation for each system. Motorhomes while on land have many of the same systems and therefore are relatable.

When it comes to trailer boats here in NZ anyway it's pretty standard to have a dual battery system, one battery to start and one battery to discharge electronics from. In motorhomes we have one battery to start the petrol engine which was factory fitted and another to discharge from for running everything behind the driver. This is referred to as the house battery or the service battery, it's normally the large deep cycle battery used to run everything and is often connected to solar panels to recharge just like in a boat.

Marine industry made the VSR or Voltage Sensitive Relay famous (also called Voltage Control Relay) as it was seen as a way to charge the house battery using diverted energy from the alternator. As time went by these got smarter to include a delay before switching to the house battery so that the start battery would charge a little first. Later versions include a parallel connection of both batteries so they would charge together. This works OK if both batteries are of similar physical size and similar state of charge but is a terrible idea when a house battery bank is upwards of twice the capacity. Even worse when the house bank is 5 times the capacity of the starting battery as they never charge correctly and the variances in voltage could take days to equalise meaning the relay will open and close repeatedly for days while switching between the two banks which can lead to the VSR failing.

Better solutions today are DC charging systems where energy is taken from one power source to charge the other. In this situation, the first battery is being recharged by the alternator and the second battery is being charged by the first battery.

Other things to consider are:

If the battery types are different do you need to use different voltages for flooded vs. AGM
If you have a lithium house bank what voltage is needed to charge but more importantly does the BMS have alerts or a cut out if something is wrong
If the lithium house bank is very large how long will the alternator running at 100% of max output
How far away from the start battery is the house battery because voltage loses should be accounted for
different types of batteries have different price points and some might be better or more suited than others

Lastly a consideration of design and purpose, batteries can be dual purpose, it's not that they are really designed that way it's more like a very large 260Ah deep cycle battery still has a 1350CCA rating so while it might be deep cycle, used primarily for a house bank doesn't mean it can't be used as a starting battery. Therefore you can simplfy a system design if you needed say 100Ah of starting battery, but 200Ah of house battery. Normally a VSR might be a good idea but what if your house battery or spare battery didn't have enough power to start your motor if your main start battery failed? This is why having a good quality house battery is important. It's your spare "get me out of jail" battery. It's not like you can hop out your boat and push start it like you can a car or motorhome. So you can build some insurance into your design and maybe rather than worrying about needing a good DC to DC charger you could use a better start battery which matches the house battery size or type (both being AGM) and therefore you have more usable power and less requirements on electronics.

Total Loss Ignition Systems

What is a Total Loss Ignition System? Its using a petrol engine without an alternator.

Effectively you are running your vehicle's electronics from your battery which includes any spark to the ignition system, gauges, lights, fans, pumps, transponder or data logger, GPS, radio, comms etc etc.

Again we'd highlight the purpose of this battery is the most critical thing to consider. Do you want to have enough cranking power to start the vehicle or do you need to drive for a few hours without a recharge? (the latter being something to consider if your alternator dies while in competition or along way from home. Its referenced on the specs of a battery as being the reserve capacity (RC). It's measured at 25 amps and simulates SLA which was considered the essential requirement to operate a vehicle some 20 years ago. (Starting, Lights, Accessories). So the 25 amps could be discharged for X number of minutes (i.e. 90RC) until there is no usable voltage remaining in the battery.

With high flow fuel pumps and water cooling systems on high power engines a fully charged 12 volt battery might only be 11.5 volts under discharge load. This is to low a voltage for the ignition system to operate effectively and it makes current draw that much higher that it is far from ideal. An alternative would be to run a higher voltage battery like our XS Power 16 volt battery. Underload the voltage would still be 14 volts which is similar to the voltage output of a high powered alternator without the overhead of the alternator drive belt loading the engine or the extra weight being carried.

This is ideal for systems like drag cars or midget vehicles used in speedway which are push started. The 16 volt batteries also make for excellent jump start batteries on 1400 plus horsepower engines used in Hydroplanes, river racers and jet sprint boats.

Marine Batteries and CCA requirements

Have you ever wondered why a 100HP outboard requires a 800CCA battery? It actually has nothing to do with 800 cold cranking amps and the 30 second discharge time at minus 18 deg centigrade.

A 100HP outboard these days has an electronic ignition system and a high torque starter motor which only draws about 150-200A. It will start in 1 second so you could actually use a 7Ah alarm battery you use on your fish finder to start the outboard. It might, however, record a low voltage warning in the engine management ECU, and that if done repeatedly will void your mechanical warranty of the outboard. The low voltage happens when you draw high current from a small capacity battery, the voltage dips but when the discharge turns off the voltage goes back up.

So this is why you are told 800CCA. A few things to note also, have you ever noticed that there are plenty of batteries just under the magic 800CCA that are $80-100 less? That's because back in the day the outboard manufacturer being local to a country had to determine what was locally available and suitable. Many times that lead to supply agreements and those agreements were made to benefit one brand over all the others with not quite enough CCA. Anyway I digress, the reason for 800CCA is that a typical battery would have to be approx 80-90Ah to achieve that kind of spec and if its 80 plus Amp Hour then even if the battery is a few years old and is half discharged, you've got some electronics on prior to turning the key and the temp is a chilly few degrees above freezing. The battery will still have enough power to start your outboard.

There you have it, a worst-case scenario used to resolve a starting point and avoid liability of being sued. (a genuine concern if you are in America)

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