Voltage Sensing Battery Isolator
I have sought a better understanding of how the modern voltage sensing battery isolator works and obtained some interesting information from Alan Johnson from Piranha off Road Products. I have reproduced the details here, for the reference of anyone who wishes it.
Now I am not specifically “pushing” the Piranha product, or attempting to criticise those who, for one reason or another, choose an alternative, or cheaper solution. But there are certain risks in a basic parallel system that people should be aware of.
I asked three specific questions:-
1. Is the primary battery disconnected while the secondary battery is receiving its charge?
2. Are the two batteries electrically separated?
3. Are they ever connected in parallel?
No, the primary battery is NOT disconnected while the secondary battery is receiving its charge.
Yes, the two batteries are electrically separated. And sometimes?? they are, sort of, connected in parallel.
To understand the concept of voltage sensing battery charging/management systems, we need to discuss a few basic ideas first.
Parallel means the two “tanks” of energy, (batteries) are connected positive to positive, negative to negative.
This creates, electrically, not two separate batteries but one bigger battery. They charge and discharge as one.
In a good parallel system, both batteries should be identical (i.e. same size, age, design and capacity).
In a Voltage Sensing system, the batteries do not parallel, they never discharge together and the charging occurs similar to parallel, but not true to the meaning of the term parallel.
1. The car starts on one battery only. (NOT Parallel)
2. The main battery must achieve a pre-determined voltage, 13.5+ volts, before the auxiliary battery is connected to the charging system (NOT Parallel)
3. When the main battery is charged, power from the alternator takes the path of least resistance, therefore into the auxiliary battery via the main battery first. (Parallel – perhaps?)
4. If the main battery drops below the threshold of 12.8 volts, the auxiliary battery is disconnected from the charging circuit. (NOT Parallel)
5. If excessive current is sensed the auxiliary is disconnected. (NOT Parallel)
6. If the auxiliary battery is damaged and will not accept a charge, the unit will disconnect the auxiliary battery. (NOT Parallel)
Voltage Sensing systems offers the following advantages to those who wish to use dissimilar batteries.
• Faster recovery time
• Use of dissimilar batteries (size, age, design and capacity, wet cell, AGM, etc.)
• Better reliability
• Longer battery life
• Surge/Spike protection
• Virtually no voltage drop across system
• Fail Safe – If the system “dies”, only the auxiliary will go flat
A basic parallel system just does not do the job. If one was to (hypothetically), disconnect the main battery completely, the modern vehicles computer management system would sense no charge from the alternator, resulting in “limp home” mode, or possibly, complete shutdown. This is not acceptable for any outback traveler and even worse, if the system did totally fail – the engine, fuel, or spark could be jeopardised.
On start up, parallel systems can “mask” the fact that the main battery is flat, or damaged, i.e. dead cell.
The auxiliary battery will do most of the work, starting, cranking and when the auxiliary is flat/depleted, the car will not start, because BOTH batteries will now be in a discharged state.