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Inverter, Battery issue


Inverter, Battery issue

by Tony Bullard » Fri Jan 31, 2014 5:51 pm


I have been having trouble with my batteries discharging rapidly which eventually got so bad I could not keep a charge on them at all. I had just had the boat completely rewired a month ago. I have tried to charge the batteries with my inverter/charger, my gas generator, and the engine. Still no fix.

So today I took all 6 of my Trojan 105s to the battery shop to have him test them.

When I came home with NO batteries in the boat, surprisingly when I started my Honda generator I got not only AC power on the AC side, but if I switched on the inverter I also had DC power on the DC side. The DC frig, lights, etc. worked just fine without any batteries. When I turned off the inverter everything DC went dead.

I thought that the DC and AC are supposed to be completely isolated from each other except for when the inverter is making AC out of DC. It appears to be turning AC into DC. It shouldn't do that should it? Could this be what is causing my battery charging, electrical problems?

Thanks for any help.

Tony Bullard
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Re: Inverter, Battery issue

by stormalong » Fri Jan 31, 2014 6:36 pm

Your inverter is also a charger. When you applied ac power to the inverter it put out a dc charge for the batteries. Since you had no batteries the charger was powering your dc loads.


Many chargers (you don't say which one you have) depend on the battery load to regulate the voltage. You could be supplying a much higher voltage to your dc system than you want. Many inverter/chargers specifically state not to apply ac to them without batteries connected.

If, after getting you boat rewired your batteries are not holding a charge something is wrong. I suggest you measure the current flow to your batteries with everything turned off. You probably have some unknown load somewhere. Either that or your charging sources are not working properly.
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Re: Inverter, Battery issue

by Auspicious » Sat Feb 01, 2014 5:25 am

I suspect Brian is correct and that your inverter is an inverter/charger. Can you confirm that?

I'm surprised the battery shop wasn't able to load test your batteries on the spot.

There are a number of likely possibilities for your power problems.

Your batteries could be dead. Load testing your batteries is the right first step.

Your charging system may not be working. What are the options? Do you have a separate battery charger in addition to what we suspect is an inverter/charger? Have you tried charging from the main engine alternator? Do you have solar or wind power generation.

You may have a parasitic load, possibly as a result of the rewire job. Finding such a thing is time-consuming but not particularly hard.
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Re: Inverter, Battery issue

by brilliantstar » Sun Feb 02, 2014 3:38 pm

Depending on age and temperature, lead-acid batteries will self discharge from 3-20% per month.

In the case of a T-105, this could be 7-45 AH per month per battery at ~9-60 milliamps.

This evolution from 7-45 doesn't happen suddenly unless the batteries are mistreated -- the most likely causes being: repeated over-voltage charging for the battery temperature, repeated excessive discharge, or repeated discharge without full recharge.

In case one, new batteries are in order. In cases two and three equalization is in order.

So, given your batteries were performing well and normally (not necessarily the same thing) before rewiring, and now they are exceeding normal or previously observed self-discharge rates after rewiring, there is little reason to suspect anything besides the rewiring. If your charger were not working, you wouldn't see them reach full charge only to lose it rapidly -- except...

Be sure the charger is OFF when checking to see if the batteries are fully charged. The charger could have failed to charge the batteries fully, but you are seeing full charge because it has been uselessly delivering +AH on the meter without the battery actually accepting them -- depends on the controller. Many charge controllers can be set to shift from acceptance to float after a certain amount of time, even if full charging has not occurred. This is to keep from dehydrating the batteries.

I suggest you charge the batteries with them disconnected from the boat's grid. Use an independent charger, as the alternator is connected to the boat's grid and see what happens.

    If the batteries recharge to the proper voltage and hold it for a reasonable time, your boat's charger/controller is at fault.

    If the batteries behave the way you describe disconnected from the grid, the batteries are at fault.

    If after successfully charging the batteries with a good charge hold, you connect them to the boat's grid and they start discharging you have a sneak circuit or a bad switch (closed when it indicates open).
Reconnect these charged batteries with the master switch OFF. If you have a sneak circuit, it may start passing current the moment you reconnect the battery...zap.

Since you haven't smelled smoke and no wires have melted through (giving a dead circuit), whatever sneak circuit rewiring has established is operating at currents within the tolerance of the wires involved. Divide your AH losses after recharge by the hours it takes them to occur to get a sense of what amperage is involved. This may help in isolating what wires are involved, or may not.

One note here. Although that sneak circuit appears to be within the wires' tolerance, it is possible to switch something on and have that situation change for the worse. Be careful.

As Dave says, hunting these down is tedious, and you will be well served with a clamp-on ammeter. It takes very few electrician hours to pay for one.

Your best bet, unless you suspect something specific, is to start at the battery cables and work outward.

We found our sneak circuit in only a month. It occurred where a new wire had been placed such that the edge of a copper grounding strip for a long gone SSB cut through the wire's insulation. The cut was invisible when the access plate was removed. Contact only occurred when the badly designed access plate was screwed back into place. The current went straight to ground through the strap, and we had no other symptoms other than apparently failing batteries. We removed and sold the SSB straps. After that heat shrink solved the problem.
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Re: Inverter, Battery issue

by elianahermione1 » Mon Jul 09, 2018 5:15 am

Your calculations seem to imply that you are assuming the inverter converts DC to AC at 100% efficiency. You may want to look up the specs on your model of inverter. Some are more efficient than others. Many consume a certain amount of DC even at low AC demand. Also the efficiency stated may only be true for its peak rated output. Another possibility is that the inverter is very sensitive and prone to shut off when the battery voltage starts to drop below 11.5 volts or something Professional Essay Writing Service UK.
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