battery problems or voltage reading problems?

[hankyc]

So, firstly you have to excuse me for being a novice when faced with electrical problems...

 

Getting back on the canal after being hooked up to mains power for our first winter and we managed to drain our leisure batteries by mistake for the first time. They are taking a very long time to charge so I thought we might have a charging problem.

 

When engine is running and I measure across the battery terminals I only get a reading of 12.8, when under load from appliances it goes down and if I use more revs it goes up, so at first I thought it might be an alternator problem as I thought with the engine running I should get a steady reading around 14v. I then checked the starter battery (which is fully charged and I took readings isolated from leisure bank) and it was getting a steady 14v so I guess the alternator is OK?

 

Is it normal to get low readings when the batteries are charging from nearly flat?

 

Also with engine off the leisure batteries voltage drops considerably under load. Is this normal with a half charged battery or have I got dogey batteries (note they are only a year old)

 

Or am I missing the point all together??

 

thanks

[Chris Pink]

12.8 is normal when charging from flat, they will riser to 14.something over the course of charging.

 

Yes, voltage will drop under load, how much is a moveably feast depending on the size of the load. Also when a battery first comes off charge its terminal voltage is artificially high due to surface charge and this will drop off rapidly under load.

 

you don't say how big your battery bank is or what output your alternator but on an alternator from more or less flat you could be looking at up to 24 hours before they are completely charged.

[nicknorman]

Depends on how the 2 banks are charged.

 

Twin alternators?

Single alternator with diode spilt charging?

Single alternator with voltage sensitive relay?

Single alternator with manual charging selector switch?

 

However, with any of the last three it seems odd that there is such a difference between the battery voltages. With the engine running, the batteries are effectively connected together so if the leisure bats are flat, the charging voltage available to the engine bat will be pulled down to much the same value as the domestic. The fact that the domestic voltage is so much lower suggests a high resistance connection somewhere, unless you have option 1 above.

 

ETA just reread your OP and see that you were getting 14v with the leisure bat disconnected. As Chris says it could be just that you have a small alternator and a large bat bank, or it could point to an alternator problem. Best to get a clamp meter and measure the charging current, then you will know what's going on.

Edited May 23, 2013 by nicknorman

[Nickhlx]

Its normal to get a lower reading from flat batteries that are on charge as this potential difference ( between the alternator regulated output voltage and the battery voltage) is what causes the current to flow. The voltage though will quickly ( minutes) come up from low, depending on capacity of alternator and how fast it is being spun. Max alternator output is likely to be at around 5000+ alternator rpm, and unlikely to be achieved "at tickover", ( depends on pulley ratio and rev range of engine). Also it would be easier on the alternator if the revs were higher as its fan will help keep it cooler, especially when working hard....

 

When charging mine from a relatively low SoC, I increase the revs until no more current is being produced, and then a bit more for "added cooling". It is a Beta 43 with an Iskra 175, and this requires revs to be at usually around 1500, after the initial charge rate (sometimes up to 200 amps) has diminished to about half that...

 

Nick

[hankyc]

12.8 is normal when charging from flat, they will riser to 14.something over the course of charging.

 

Yes, voltage will drop under load, how much is a moveably feast depending on the size of the load. Also when a battery first comes off charge its terminal voltage is artificially high due to surface charge and this will drop off rapidly under load.

 

you don't say how big your battery bank is or what output your alternator but on an alternator from more or less flat you could be looking at up to 24 hours before they are completely charged.

 

thanks so much, that is exactly what I needed to know, I thought it would be my ignorance rather than a problem but hey I'm still learning!