Is your controller A new model Varistream. If it is then L = low voltage and U is a voltage below 10.3v where the controller cut the power to save the battery.
I once noted this on a different thread with regard to charging a battery with a SCR.
"I have a 110 amp leisure battery powering my system with a split charge relay.
I have this in a Citroen Relay SWB van with a 150amp alternator.
The reason why we have to charge our leisure batteries regularily is that we mostly don't drive enough miles to put back into the battery what we take out.
Each of my Shurflo pumps draws about 3.4amps with the digital controller setting we use. The battery will accept a recharge of about 10 amps at best, but usually between about 5 - 7.5 amps with a fairly full battery - remember a battery nearing a fully charged state will accept an ever reducing charge until the battery is fully charged.
So if we travel 10 minutes to and from the day's work, a 20 minute charge of 7.5amps ( = 2.5amps) won't come anywhere near replacing the 13.5 amps we have used assuming the pump runs 4 hours.
The split charge relay wiring from the main van battery is protected by a 15 amp fuse which has never blown in the 3 years (now 5 years) this system has been in this van. This means that the amps drawn by the leisure battery has never been more that 15 amps even when the battery was flat. If it had then the fuse would have blown.
My son in law has a Ford Transit Connect LWB with the latest Varistream digital controller and he drives around trip of about 30 miles a day and he now manages keep his 85amp leisure battery pretty much fully charged travelling this distance with his split charge relay. (The new digital Varistream draws current even when the controller is switched off, and its only since we have put an isolator switch on the supply to the Varistream that we have been able to achieve this. We estimate that the draw on the battery was about 25 amps over a week.) But it's not the distance he travels that's the most important - what is important is how long it takes. I have found that my battery will still be charged at the exact same rate whether the engine is reving at 3000 rpm or just idling at traffic lights."
Here is another post with more detail about charging and fitting an isolator switch.
"......... you need to order or buy a 10 amp on/off switch from an electrical sales outlet. This will be fine.
http://www.screwfix.com/p/lap-1-gang-10a-switch-ip55-240v/47152Now you want to put this next to your controller so you can switch off the current from your leisure battery to your Varistream. So this will be an isolator switch. The problem with the latest Varistream is that they use current even when they are switched off.
We found that a new fully charged 85 amp Numax leisure battery was being drained to a 75% charge just lying idle for a week. At the end of the day just switch the switch off - end of story.
The very reason you are able to let the van engine run and charge the leisure battery a bit shows that the split charge relay system is working correctly.
I have fitted a volt and amp meter to my system to see what actually happens during a normal working day.
With our Varistreams the higher the pump flow rate the more current (amps) the motor draws. So an unregulated pump running flat out would be drawing about 8 amps, whereas ours will be drawing around half that, depending on our flow rate setting.
If we assume that your pump is drawing 4 amps per hour and your pump actually pumps water for 4 hours through the day, then the pump will have drawn 16 amps of current from the battery. In theory this means that if you have an 85 amp battery you will only have 69 amps left.
Even when my battery is half charged my alternator settles down to a charge rate of 8 amps when recharging the leisure battery. In theory this means that your van needs to be charging your battery for 2 hours to replenish what you took out. (Unfortunately it isn't as simple as that as the battery will only accept a smaller and smaller charge as it gets fuller. I once read in an Australian camper van article that someone estimated it would take a road journey of 750 kms to recharge an 85amp leisure battery via the vans alternator.)
Now it usually takes me 10 minutes to drive to my first house maybe another 10 minutes moving the van etc and 10 minutes driving home. So using the above example, I am taking 16 amps out of the battery and only putting 4 amps back in the day (30 minutes driving at a recharge of 8 amps). So you can see that within a short period of time my battery would be flat if I didn't supplement it's charge with my battery charger.
If I get stuck in traffic, thats good for my leisure battery as the van still charges my battery at the same rate idling as it does at 3000 rpm on the motorway. So the longer it takes you to get home, ie the slower your drive, the more charge you are putting into your leisure battery and also saving fuel.
If you do a search, I have commented on leisure battery charging on numerous occasions, some of which are relevant to you."
Hope that helps.
Adding to this;
My leisure battery was showing a charge of 75% of full charge about 4 months ago. I embarked on a journey to London in my Citroen Relay van. It took about 4 hours of driving (around 250 miles) before the leisure battery was fully charged (14.4 volts with a trickle charge of around half an amp. The initial charge rate into the leisure battery on setting off was 4.9 amps at 13.8v.)
