I got two chargers, one old one, charges on boost 8amps or trickle at 2 amps.
I hook up a champion lead acid battery, all good, charging on both settings. Fine.

I also got a 'smart charger' which does not recognise the one year old lead acid battery. It is on the right setting.I can bend over backwards, to no avail.
It was charging the boat's AGM's but always at a slow rate. No way l could induce the thing to charge full boost though the AGM' could take it.
Because l am on solar power, l am not worried although it is a mystery.

So you are to blame Trek! Next time please amake me a 40amp battery charger and skip the trickle 10amp 6 hour start up I just want the full 40 amp charge just like alternators do every start up!

Trek said..
And that will only work if the manual matches the software in the battery charger in question. I know this because I design battery chargers and their software as a sideline and the software varies hugely depending on what the customer for the battery charger wants and the type of battery/s.

ah ha! an expert!

i have been considering the pros and cons of having a DC-DC charger on board and am leaning towards not adding another piece of electronic equipment that might fail.

is it also true that a smart charger cannot correctly determine the appropriate charge rate if the batteries are currently under load?

fishmonkey said..

Trek said..
And that will only work if the manual matches the software in the battery charger in question. I know this because I design battery chargers and their software as a sideline and the software varies hugely depending on what the customer for the battery charger wants and the type of battery/s.

ah ha! an expert!

i have been considering the pros and cons of having a DC-DC charger on board and am leaning towards not adding another piece of electronic equipment that might fail.

is it also true that a smart charger cannot correctly determine the appropriate charge rate if the batteries are currently under load?

Yes often true. A big problem is the battery charger is sensing what voltage the battery is from down the wrong end of its cables which may/may not have the charge current running through them. The voltage drop across those cables at 40A can easily be 0.5V or more.

While charging that means the charger will see an artificially high voltage because that 0.5V will add onto the battery volts. So a 12.1V half dead battery will "apparently" read 12.6V and the charger might think it the battery doesnt need charging. But some charger software (mind does) alters the charge current to estimate the remaining battery capacity and how much to charge. The trouble with that is the actual battery voltage at its terminals rises during charging indicating a false "higher remaining capacity" as well, and vice versa under load...... and that changes depending on the temperature of the day .... Then the type of materials used to make the battery cause all those voltages to differ as well.... I am losing my hair Southace!

A VSR is simpler and probably a much cheaper option than a DC-DC charger, and it would suit most situations. Add an emergency parallel switch for starting with a dead starter battery.

However if your two batteries have different preferred charging voltages or profiles, a VSR may not be suitable. This may be the case if, for example, you have a 12V starting battery and 24V house; or you have a lead-acid starting battery and LiFePO house. A DC-DC charger may then be the way to go.

Although maybe not ideal, I've used a VSR, normal wet cell lead-acid starter battery, and different chemistry house (Gel and AGM at different times) with good results - optimise your charging for the house, and the starting battery will probably cope OK.

BEP have high quality VSRs and switches in various configurations, but they're expensive. Here are some cheap ones:
www.australiandirect.com.au/Battery-Systems/Battery-Chargers/KAVSR
www.jaycar.com.au/Power-Products-Electrical/Storage-Batteries/Battery-Accessories/140A-Dual-Battery-Isolator-%28VSR%29/p/MB3685

You can usually minimise problems with voltage drop by ensuring your wire guage is high enough, checking all terminations are clean and tight, and if possible making sure your charger senses the battery voltage directly (that is, have separate sensing wires going direct to the battery terminals).

DC-DC chargers have come about mainly due to the fact that some car alternators now switch off charge to save fuel - this means that if you want to run a house battery it will not charge effectively.

This also can assist when charging a battery some distance from the source battery such as for a bow thruster or windlass - as the voltage drop in the cable is made up by the DC-DC charger and the battery is therefore charged effectively.

For boats, none that i know of have alternators that switch off. if the batteries are in the same battery box, voltage drop is negligable. if they are different voltages or chemistries then, yes DC-DC chargers are an option.