Batteries lead an isolated life

Battery isolators and battery systems are always an area of hot debate. This article is not to get into the differing battery constructions and variants, but rather into the systems to manage them.

In an ideal world every cell would be managed independently, and batteries would cover all our needs. The reality is that we have to make do with  differing solutions and varied information.

Some of the best battery information is here, here and here

This guy also has great information.

The general rule is that deep discharges shorten a Lead Acid batteries life. Cells not built for this are damaged very quickly. My experience has been that an average brand name 12mth old battery flattened by headlights overnight is basically stuffed and won’t carry more than 30% of it’s new capacity. It’ll start the car fine with this shallow charge, but that’s it.

The goal of an isolator system should be to
a) Leave you able to start the car
b) Minimise deep discharges of any battery

Now we come to the point of most arguments discussions. In most cases people opt for separate batteries, one for starting and one for auxiliary use. This means that the requirements the batteries face are not the same, and often differing batteries are chosen for these tasks. Further, as they are subjected to differing conditions they age independently and often will be replaced at different times. Yet the manual that your isolator came with, your mate on the Internet, and quite a few battery manufacturers will be telling you “DO NOT PARALLEL BATTERIES OF DIFFERENT CONSTRUCTION / SIZE / AGES”. That’s it. The discussion never moves beyond that point. There are HUGE debates on forums all over the Internet, yet they can’t get beyond this point. Series string construction and management is discussed ad-nauseam, yet the classic 4wd warning remains. Luckily for us, they are WRONG.

Evidence, More Evidence, even More Evidence

Parallel what the hell you like, it won’t matter. Which is lucky, because MOST expensive battery isolators do just that, and have for years. They all claim to be magic in a box, but most are relays, something known about for a little while now and use in all sorts of places.

Battery Calcs & Types

The calculations below are based on what I phrase “effective capacity”. This varies depending on battery type and it’s use. I’m going to assume you have it in a 4wd and drive a 2 – 4 hours a day when on trips. The numbers below are based on not wanting to flatten the battery more than 80% and it’s likely state of recharge from a normal alternator (your mileage may vary). You can roughly work out your effective capacity as follows.

1. Starter Battery
   (0.4 * RC) * 0.8
   =approx 48Ah for N70ZZ batt
2. Deep Cycle
   Ah * 0.5
   =approx 50Ah for 100Ah batt due to common low state of charging.
   This *may* be better if there are very long charge times or high charge voltages.
3. AGM
   Ah * 0.8
   =approx 80Ah for 100Ah batt due to fast charge capability.

Remember – these are for NEW batteries – it only gets worse as they age. Ageing depends on time, temperature, cycles and depth of discharge.

Isolator System Types

1. Parallel / Single Battery
   Works well enough, if you don’t discharge it too far. If you do discharge it you’ll be stuck looking for a jump start. Auto cut out devices and voltage monitors are a good solution. It’s also the cheapest up front solution – no parts needed. You’ll not be game to discharge your batt’s below 70% effective capacity if you want to start the car, so your capacity is:
   0.7
   * “effective Ah rating of battery type”
   * 2
   =Useable AH
2. Diode Isolator
   These are generally regarded as inefficient. Jaycar has one here. They induce a 0.6v drop across them. According to Odyssey this is the difference between fully charged and 50%. What will happen is charge voltages are high enough that losing 0.6v means the battery will charge 30% – 50% more slowly than normal, and possibly not charge fully. Theoretically a solid state MOSFET solution would work well, but I haven’t seen one yet. Relays are cheaper I guess. You only get one battery so the rough calculation is:
   AH = 0.7 * “effective Ah rating of Aux Batt Type Only and a slow charge”
3. Voltage Regulated Isolator
   This is the most common solution. Redarc is the most obvious (no box), but Piranha’s black box of tricks is the same thing. My black box fell to bits and inside it’s just relays connecting the batteries. They work by isolating the batteries when the voltage drops below a set point – around 13v. When the primary battery is back above 13v due to the engine running, it reconnects them. They generally should build in a bit of hysteresis (damping) to stop is short cycling if the voltage is close to the cutoff. The calculation is:
   AH = “Effective Ah rating of Aux Batt Type”
4. VR Isolated with Lower Cutout than Cut-in.
   Traxide does this really cool VR Relay isolator. By setting the cutout to the mid 12v, it lets you use 50% of your primary battery. Normally in an isolated system there is this huge heavy main battery there for just starting. I find this a waste. If I can use 50% of it’s capacity safely, then I would prefer to do so. The calculation for this system is:
   AH = (“Effective Ah rating” (Aux)) + (0.5*”Effective Ah rating” (Main))
5. Independent Management
   Now we get to the tricky (read expensive) stuff. To fully manage all batteries to their maximum potential all batteries (and ideally cells) need independent attention. People that live on solar or independent power often take inordinate care of their batteries, or alternately buy batteries that last 50 years. For 4wd use this means independent charging. There are a number of ways this can be achieved from vendors such as Rotronics, Amplepower and Arrid. Temperature compensation is one of the most important components, but they also regulate charge voltage, pulsation, float test etc etc. Nice stuff, but be prepared to pay for it. It may be cheaper for the casual user just to put new batteries in every 2 – 3 years, but if you live off it, then your use would probably justify the extended service life.

Personally I work out the following.

Battery & Isolator Cost and Capacity Calculator Spreadsheet

My Waeco CF50 uses *about* 3A @ 50% duty cycle * 24Hrs = 36Ah/day.
Add a couple of 8w flouro’s (1.5A * 3Hrs) and maybe the radio = about 45Ah / day.
I often want to stay 2 nights in the one spot, so I want nearly 2 days supply.
80 – 100Ah is enough for what I need.
The cheapest system that can deliver that is a Normal Start Battery + an AGM with a Traxide isolator.

There is a spreadsheet attached here that works out approximate the economics and capacities of each system. If you have unusual requirements or conditions, then the numbers will have to be tweaked a bit.

Interestingly the cheapest setup that also gives the most Ah/$ is simply a pair of normal Start batteries in Parallel. Unfortunately 67Ah useable is not enough capacity for my needs. $4.50/Ah

The most cost effective for my requirements is a normal Start battery + an AGM with a “Tricky Voltage Relay” by Traxide. 104Ah and $5.30/Ah

The very common Isolator + Deep Cycle solution works out to about $11.0/Ah – not very cost effective.