Auxiliary Battery Charger / Maintainer


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Save money on batteries and stop them from being destroyed by discharging. a flat battery means you need an expensive replacement.
The auxiliary battery charger maintainer is simple to install, low cost product designed to enable extra battery banks on a boat or a camper van to be kept topped up from a separate battery bank which has, for example, a battery charger / wind gen/ solar cell connected to it.
Why use this product instead of a voltage sensitive relay?
A voltage sensitive relay is great where a large current flow is required, but it takes a relatively large amount of power to hold in a relay, this is power you cannot afford if you are only producing low current, also if the other battery you are charging is a engine battery then the starting current that will be present on engine start up will cause problems through the relay unless its current limiting. the battery maintainer uses fets instead or relays and as such uses very little power itself (less than 1.5 ma) and so the power generated for example in a solar cell ) can be passed through with little loss. also the maintainer is current limiting so the high load on starting would not affect it. relays also have a limited life cycle and on low power generation a relay could be clicking on and off all day, however with a fet control system does not have any physical contacts to wear out and no noise produced.
How does it work?
This unit is activated when the main battery has reached about 13.3 volts and allows excess power to be transferred from the primary charging system to charge/maintain an auxiliary battery bank.
Please note that this product has no fast charge / boost ability but simply mirrors the input voltage with a small voltage drop (depending on the current flow), the final charge voltage would be around 0.2 volts lower than the primary charge voltage, more than enough to keep a battery topped up and supply any small loads such as the battery internal discharge and the engine management system discharge (up to about 1 amp continuous). this product should not be used on a battery with a sustained heavy discharge or drain as it does not have the ability to pass high currents, for higher currents see the voltage sensitive relays or battery to battery chargers etc in the catalogue.
This product has the ability to charge and maintain the charge, even if the battery is low (in a long time frame for example a 100 amp battery at 50% discharge could take about 1-2 days) it would never do this if the current discharges from that battery is in excess of 2 amps.


Examples of where one would use this type product successfully.
  1. boats, if there is only 1 charger on board on the domestic system (such as a combi inverter charger) then this could be used to connect from the domestic battery bank to the generator battery, or engine starter batteries to maintain their charge state when the combi charger is on. or you may have a 24 v main battery but a 12 v generator, or a 12 v main battery with a 24 v gen (the list is endless)
  2. camper vans, where the camper van has a mains battery charger on the domestic battery but none on the engine start battery; if the vehicle is left for over a few weeks the engine start battery can be discharged through the engine management ( e.c.u.) system or if a tracker is fitted, this will also drain the system. the product will keep the starter battery charged and maintain a reasonable current to cover the losses due to the electronics on the engine starter battery and keep the vehicle in a ready condition ( as long as the battery charger is on the domestic battery system, or there is surplus power provided by a solar cell/wind turbine). maybe your camper van has a small generator on it, or the vehicle is 24 v and the gen battery is 12 v.
  3. solar / wind top up, you may have a solar cell or a wind turbine on the main battery bank and again wish to share the surplus power from this device to other batteries , to maintain them at a ready condition
  • Power consumption in off line/standby 0.001 amps approx 1 amp hr every 40 days
  • Power consumption on line (normally not important but only important if a low power solar cell is being used) = 0.015 amps so a solar cell in excess of 1 watt would work through this.
  • High voltage trip 15 v on aux battery bank (x 2 for 24 v), high temp lock down at 80 deg c
  • Aux battery low voltage warning led on if aux bat below 12.6 off above 12.7 volts
  • Voltage required to activate the device 13.3 v input
  • Voltage at which point the device switches off and on to standby 12.9 v input
  • Reverse polarity protected (fuse)

input v d/c
output v d/campssize l x w x d mmweight kgpart nos
12 12 3 140 x 45 x 40 0.025 bm12123
12 24 1 140 x 45 x 40 0.025 bm12241
24 24 1 140 x 45 x 40 0.025 bm24241
24 12 1 140 x 45 x 40 0.025 bm24121

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