Case Study - VSR or DC to DC charger
Well that will depend on your situation. We recently advised a client on the benefits of DC to DC charging because he highlighted a problem with his existing system in a boat they just purchased. The system had a start battery (N150) a service battery for house loads (to run equipment like fridge, lights etc) and a battery up-front (bow) with the anchor/winch. The system used a DVSR between the start battery and house battery. The start battery circuit also had an isolated single smart output VSR with Low Voltage Disconnect so it didn't flatten the start battery. (essentially features of a VSR but marketed to suit the sales purpose). The customer said if the engine was running it potentially kept all 3 batteries charged.
Lets work this out; If the start battery was an N150 with 184Ah, it might have only needed a 15 minutes charge at 13.6 volts to recharge from one engine start. It is however connected to the anchor battery which was a 100Ah and is over 5 metres away from the start battery which was close to the alternator. We know that there will be voltage drop over that distance, a higher internal resistance of the battery and the added resistance of the cables over 5 metres. There is no way the alternator can differentiate between the two batteries needs. It provides a fixed voltage and the same current to both batteries. Equalisation may occur if the banks are connected for 24 hours but we know they are only connected while the VSR is active and the engine is running.
The house battery which is parallel connected to the house battery using a Dual sensing Voltage Sensitive Relay (DVSR) in this instance was another 100Ah battery. So parallel with a starting 184Ah start battery). They are never going to be the same state of charge so let's just assume the house battery will have a current discharging from it while being used. The alternator will supply a float running voltage of approx 13.X volts. That voltage is required to charge a flat or discharged house battery with its low internal resistance because of its discharge. But its got in parallel with it a twice the capacity almost fully charged start battery with high internal resistance. The differences in capacity and resistances will cause the low supply of current from the alternator. The low current and low voltage will slow charge the house battery meaning it may not ever reach a full charge unless you run the onboard engine for 8 plus hours a day giving both batteries time to equalise and the batteries to finish the absorption and or float phase of charging a battery.
The solution: the installation of a DC 2 DC charger on the house battery where the voltage can be ramped up to meet the demands of the charging profile required to fully recharge a potentially flat house battery. The customer increased the battery capacity to offer a longer time between charging requirements but this wouldn't of been possible if some form of regulation was used as charging at 14.7 volts is 50% more efficient than charging at 13.X volts from an alternators output. It means the house battery with its larger capacity has a regulated voltage that's matched to the battery type (14.4v for flooded or 14.7v for AGM or otherwise) so best maintain and support a healthy state of charge.
The start battery is no-longer paralleled to the house battery because the DC 2 DC units isolates the batteries ensuring the capacity and state of charge isn't messing with the absorption of energy into the start and now that is inline with the bow battery which is only used with the engine running the current requirement is met by the alternator. Later the client may add another DC 2 DC for that battery but it raises the other question.
Do I need a DC 2 DC charger for the extra dollars it will cost if the secondary battery is less money than the difference between a DC charger and a VSR?
Well, that's a question only you can answer, but with DC chargers also managing the chemistry type, or being able to select a custom charge profile that suits your situation it doesn't take long to make the number work whichever way you need to justify on way or another and that's your call.